feat: task 2.4 WS client + rAF coalescer + Zustand position store

Eight files under src/live/:
- constants.ts: throughput-discipline numbers (MAX_TRAIL_LENGTH=200,
  reconnect backoff [1s/2s/4s/8s/16s, 30s ceiling], STALE_CONNECTION_MS).
- protocol.ts: zod discriminatedUnion('type', ...) for inbound
  (subscribed / unsubscribed / position / error). PositionEntry +
  SubscribeResult types per processor-ws-contract.
- connection-store.ts: Zustand store with status state machine.
- position-store.ts: Zustand store with latestByDevice + trailsByDevice
  Maps, applySnapshot/applyPositions/clearForEvent/selectDevice
  actions. Ring-buffer cap on trails; same-coordinate dedup.
- coalescer.ts: ~30-line rAF coalescer. Per-device buffer; flushes once
  per animation frame regardless of receive rate.
- ws-client.ts: state machine (idle / connecting / connected /
  reconnecting / closed) with exponential backoff, re-subscribe on
  reconnect, stale-connection check, subscribe correlation via
  pending-map + 5s timeout. URL resolution helper toAbsoluteWsUrl()
  for same-origin path inputs.
- bootstrap.tsx: <LiveBootstrap> creates the client when authenticated,
  wires positions through the coalescer to the position store, tears
  down on logout. getLiveClient() exposes the singleton for 2.7.
- index.ts: barrel re-exports.

main.tsx wraps <App /> in <LiveBootstrap> alongside <AuthBootstrap>.

Deviations:
1. Skipped the setStatus helper inside createLiveClient; conditional
   Parameters<> generics were hostile. Direct
   useConnectionStore.getState().setStatus(...) at the ~6 call sites.
2. subscribe() adds to the subscriptions Set even when not connected
   (so it replays on reconnect). Caller handles 'not-connected' by
   waiting for connection-store status transition.
3. onPosition returns an unsubscribe fn (Set-based). Multi-handler is
   free; lets future debug panels/tests attach.

Bundle: src/live/ adds ~15KB raw to the main bundle (mostly zod's
discriminated-union runtime). Total 393KB / 120KB gz.

.planning/phase-2-live-map/03-sprite-preload.md

@@ -156,10 +156,11 @@ For 2.3, just include it in the registry and it'll be there when 2.5 needs it.
 
 **Deviations from spec:**
 
-1. Spec sketched the direction sprite as part of the same composition (background + tinted icon). Implemented as two separate sprite types: category sprites have the plate, direction sprites are the arrow alone (no plate). Reason: the direction sprite is rendered as a *separate* symbol layer in 2.5, overlaid on top of the device sprite — drawing a plate under the arrow would create a double-plate visual. The spec's example expression `'icon-image': '{category}-{color}'` for one symbol layer + `'icon-image': 'direction-{color}'` for the direction layer is what 2.5 will actually consume.
+1. Spec sketched the direction sprite as part of the same composition (background + tinted icon). Implemented as two separate sprite types: category sprites have the plate, direction sprites are the arrow alone (no plate). Reason: the direction sprite is rendered as a _separate_ symbol layer in 2.5, overlaid on top of the device sprite — drawing a plate under the arrow would create a double-plate visual. The spec's example expression `'icon-image': '{category}-{color}'` for one symbol layer + `'icon-image': 'direction-{color}'` for the direction layer is what 2.5 will actually consume.
 2. Spec showed sample colours as `success: 'green'`. Used the design system's actual semantic palette (`#2E8C4A` green, `#E8412B` race-flag red, `#0E0E0C` ink, `#2563C8` info blue) directly. When 3.8 lands, these get rebound to TRM design tokens via CSS variables.
 
 **Smoke check (local `pnpm dev`):**
+
 - App boots; `getSpriteRegistry().size` returns `32` after the page settles.
 - `/monitor` map renders; switching basemaps doesn't break sprites (visible via the mapReady flow's "preload then install" sequence in dev tools console).
 - No "Image with id X is missing" warnings.

.planning/phase-2-live-map/04-ws-client-and-position-store.md

@@ -361,4 +361,32 @@ Mount inside `<AuthBootstrap>` so it only connects when authenticated. On logout
 
 ## Done
 
-(Filled in when the task lands.)
+Eight files under `src/live/`:
+
+- **`constants.ts`** — `MAX_TRAIL_LENGTH=200`, `RECONNECT_BACKOFF_MS=[1s, 2s, 4s, 8s, 16s]`, `RECONNECT_CEILING_MS=30s`, `SUBSCRIBE_TIMEOUT_MS=5s`, `STALE_CONNECTION_MS=60s`. Single source of truth for the throughput discipline.
+- **`protocol.ts`** — zod schemas for inbound (`subscribed` / `unsubscribed` / `position` / `error`) via `discriminatedUnion('type', ...)`, `OutboundMessage` type union, `PositionEntry` / `SubscribeResult` types per [[processor-ws-contract]].
+- **`connection-store.ts`** — Zustand store with `status` / `attempt` / `lastConnectedAt` / `lastErrorMessage`. `setStatus(status, opts)` updates with sensible defaults (resets `attempt` on `'connected'`, stamps `lastConnectedAt`).
+- **`position-store.ts`** — Zustand store with `latestByDevice: Map`, `trailsByDevice: Map`, `selectedDeviceId`, `activeEventId`. Actions: `applySnapshot` (wipes + seeds), `applyPositions` (per-device update with same-coordinate dedup; ring-buffer cap via `MAX_TRAIL_LENGTH`), `clearForEvent`, `selectDevice`. Each update creates a new `Map` reference so Zustand selectors fire only for changed devices (Map.get returns the same reference if the entry didn't change).
+- **`coalescer.ts`** — `createCoalescer(onFlush)` with `push` and `cancel`. ~30 lines. Per-device buffer; `requestAnimationFrame` flushes the latest snapshot once per frame; `cancelAnimationFrame` on cancel. The throughput-discipline core.
+- **`ws-client.ts`** — `createLiveClient({ url })` returns a `LiveClient` with `connect / close / subscribe / unsubscribe / onPosition`. State machine: `idle` / `connecting` / `connected` / `reconnecting` / `closed`. Reconnect with exponential backoff capped at 30s. Re-subscribes to all active topics on reconnect. Stale-connection check via `setInterval` halves `STALE_CONNECTION_MS`; closes if no message in window. Subscribe correlation via 5s-timeout pending map. URL resolution helper `toAbsoluteWsUrl()` derives `ws(s)://...` from same-origin path inputs (`/ws-live` → `ws://localhost:5173/ws-live`).
+- **`bootstrap.tsx`** — `<LiveBootstrap>` React component. Watches auth status; creates the singleton client + coalescer when `'authenticated'`, tears down on any other status (closes WS, cancels coalescer, calls `clearForEvent` on the position store). `getLiveClient()` exposes the singleton for the event picker (2.7).
+- **`index.ts`** — barrel re-exports.
+
+**`src/main.tsx`** updated — `<LiveBootstrap>` wraps `<App />`, sandwiched between `<AuthBootstrap>` and the route tree. Connection only happens for authenticated users.
+
+**Deviations from spec:**
+
+1. Spec sketched a `setStatus` helper inside `createLiveClient` for shorter call sites. Tried it; the conditional-types-on-Parameters generic was hostile. Switched to direct `useConnectionStore.getState().setStatus(...)` call sites — verbose but readable, and there are only ~6 of them.
+2. `subscribe()` returns `{ ok: false, code: 'not-connected' }` if the client isn't currently connected, **but still adds the topic to the `subscriptions` Set** so it gets replayed when the WS opens. Spec sketched `subscriptions.add` only on success; the replay-on-reconnect contract works whether the user's first subscribe fired during a connecting state or a connected state. Document expectation: caller (event picker in 2.7) handles `not-connected` by treating it as "subscription will activate when the connection comes back" — typically by listening on the connection-store's `'connected'` transition.
+3. `onPosition` returns an unsubscribe function (Set-add + delete). Spec showed it as a single-handler API; multi-handler support is no extra cost and lets future code (a debug panel, tests) attach without fighting the singleton handler.
+
+**Smoke check (local `pnpm dev`):**
+- App boots; no WS connection until login.
+- After login: `useConnectionStore.getState()` shows `status: 'connecting'` then either `'connected'` (if Phase 1.5 stage processor is reachable) or `'reconnecting'` with backoff (if the proxy isn't routing `/ws-live` yet).
+- `usePositionStore.getState()` shows the empty initial state — `activeEventId: null`, empty Maps.
+- Logging out closes the WS and clears the position store.
+- DevTools network tab: WS handshake to `ws://localhost:5173/ws-live` after login; carries the auth cookie automatically (same-origin).
+
+**Bundle:** `src/live/` adds ~15KB raw (the bulk is zod's discriminated-union runtime, plus the WS state machine). Loaded eagerly because `<LiveBootstrap>` is in `main.tsx`. Total main bundle 393KB / 120KB gzipped (up from 376KB / 115KB after 2.3).
+
+Landed in `PENDING_SHA`.

.planning/phase-2-live-map/README.md

@@ -51,7 +51,7 @@ When Phase 2 is done:
 | 2.1 | [MapView singleton + mapReady gate](./01-mapview-singleton.md)                             | 🟩     |
 | 2.2 | [Tile-source switcher](./02-tile-source-switcher.md)                                       | 🟩     |
 | 2.3 | [Sprite preload (racing categories)](./03-sprite-preload.md)                               | 🟩     |
-| 2.4 | [WS client + rAF coalescer + Zustand position store](./04-ws-client-and-position-store.md) | ⬜     |
+| 2.4 | [WS client + rAF coalescer + Zustand position store](./04-ws-client-and-position-store.md) | 🟩     |
 | 2.5 | [MapPositions (clustered + selected sources)](./05-map-positions.md)                       | ⬜     |
 | 2.6 | [MapTrails (bounded ring buffer, polyline rendering)](./06-map-trails.md)                  | ⬜     |
 | 2.7 | [Event picker (subscription driver)](./07-event-picker.md)                                 | ⬜     |

src/live/bootstrap.tsx

@@ -0,0 +1,65 @@
+import { useEffect, type ReactNode } from 'react';
+import { useRuntimeConfig } from '@/config/context';
+import { useAuthStore } from '@/auth';
+import { createCoalescer } from './coalescer';
+import { usePositionStore } from './position-store';
+import { createLiveClient, type LiveClient } from './ws-client';
+
+let _client: LiveClient | null = null;
+
+/**
+ * Read-only access to the singleton live client. Used by the event picker
+ * (2.7) to call `subscribe`/`unsubscribe`. Throws if `<LiveBootstrap>`
+ * hasn't mounted yet — that should only happen when the user is not
+ * authenticated, in which case there's no live data to consume anyway.
+ */
+export function getLiveClient(): LiveClient {
+  if (!_client) {
+    throw new Error(
+      'getLiveClient() called before <LiveBootstrap> mounted (or while unauthenticated).',
+    );
+  }
+  return _client;
+}
+
+/**
+ * Wires the WS client + coalescer + position store. Mounts once,
+ * inside the auth-bootstrap shell, and creates/destroys the client based
+ * on the auth status:
+ *
+ * - `'authenticated'` → create the client, connect, wire positions to
+ *   the coalescer which feeds the position store.
+ * - any other status → tear down (auth was lost; close the WS).
+ *
+ * No subscriptions are made here. The event picker (2.7) is responsible
+ * for `client.subscribe('event:<id>')` once the user picks an event.
+ */
+export function LiveBootstrap({ children }: { children: ReactNode }) {
+  const cfg = useRuntimeConfig();
+  const status = useAuthStore((s) => s.status);
+
+  useEffect(() => {
+    if (status !== 'authenticated') return;
+
+    const client = createLiveClient({ url: cfg.liveWsUrl });
+    _client = client;
+
+    const coalescer = createCoalescer((snapshot) => {
+      usePositionStore.getState().applyPositions(snapshot);
+    });
+    const off = client.onPosition((msg) => coalescer.push(msg));
+
+    client.connect();
+
+    return () => {
+      off();
+      client.close();
+      coalescer.cancel();
+      _client = null;
+      // Drop any device-keyed state from the previous session.
+      usePositionStore.getState().clearForEvent();
+    };
+  }, [status, cfg.liveWsUrl]);
+
+  return <>{children}</>;
+}

src/live/coalescer.ts

@@ -0,0 +1,51 @@
+import type { PositionEntry } from './protocol';
+
+export interface Coalescer {
+  /** Buffer the latest position for `deviceId`. Replaces any earlier in-flight value. */
+  push: (p: PositionEntry) => void;
+  /** Drop the buffer and cancel any pending flush. Used on event-switch and shutdown. */
+  cancel: () => void;
+}
+
+/**
+ * Per-frame coalescer at the WebSocket boundary.
+ *
+ * Every incoming `position` message lands in a per-device buffer; the
+ * latest wins. One `requestAnimationFrame` tick flushes the snapshot to
+ * the consumer (typically `usePositionStore.getState().applyPositions`).
+ * That caps the dispatch rate at the browser's frame rate (~60 Hz)
+ * regardless of how fast positions arrive.
+ *
+ * This is the discipline traccar-web lacks: per-message Redux dispatch
+ * cascades through selectors and rebuilds full feature collections at
+ * every position arrival, which is the most likely cause of its observed
+ * lag at high update rates.
+ */
+export function createCoalescer(onFlush: (snapshot: PositionEntry[]) => void): Coalescer {
+  const buffer = new Map<string, PositionEntry>();
+  let rafId: number | null = null;
+
+  function flush(): void {
+    rafId = null;
+    if (buffer.size === 0) return;
+    const snapshot = Array.from(buffer.values());
+    buffer.clear();
+    onFlush(snapshot);
+  }
+
+  return {
+    push(p) {
+      buffer.set(p.deviceId, p);
+      if (rafId === null) {
+        rafId = requestAnimationFrame(flush);
+      }
+    },
+    cancel() {
+      buffer.clear();
+      if (rafId !== null) {
+        cancelAnimationFrame(rafId);
+        rafId = null;
+      }
+    },
+  };
+}

src/live/connection-store.ts

@@ -0,0 +1,34 @@
+import { create } from 'zustand';
+
+export type ConnectionStatus = 'disconnected' | 'connecting' | 'connected' | 'reconnecting';
+
+type ConnectionState = {
+  status: ConnectionStatus;
+  /** Current reconnect attempt counter; reset to 0 once connected. */
+  attempt: number;
+  /** Wall-clock timestamp of the most recent successful connection. */
+  lastConnectedAt: number | null;
+  /** Most recent error message from the connection layer (logged + surfaced via the chip). */
+  lastErrorMessage: string | null;
+};
+
+type ConnectionActions = {
+  setStatus: (status: ConnectionStatus, opts?: { attempt?: number; error?: string | null }) => void;
+};
+
+type Store = ConnectionState & ConnectionActions;
+
+export const useConnectionStore = create<Store>((set) => ({
+  status: 'disconnected',
+  attempt: 0,
+  lastConnectedAt: null,
+  lastErrorMessage: null,
+
+  setStatus: (status, opts) =>
+    set((prev) => ({
+      status,
+      attempt: opts?.attempt ?? (status === 'connected' ? 0 : prev.attempt),
+      lastConnectedAt: status === 'connected' ? Date.now() : prev.lastConnectedAt,
+      lastErrorMessage: opts?.error !== undefined ? opts.error : prev.lastErrorMessage,
+    })),
+}));

src/live/constants.ts

@@ -0,0 +1,19 @@
+/** Maximum number of positions kept per device in the trail ring buffer. */
+export const MAX_TRAIL_LENGTH = 200;
+
+/** Reconnect backoff schedule. Index = attempt number (1-based, with -1 → first entry). */
+export const RECONNECT_BACKOFF_MS = [1000, 2000, 4000, 8000, 16000];
+
+/** Backoff cap applied for any attempt beyond `RECONNECT_BACKOFF_MS.length`. */
+export const RECONNECT_CEILING_MS = 30000;
+
+/** How long to wait for a `subscribed` / `unsubscribed` / `error` reply before failing the promise. */
+export const SUBSCRIBE_TIMEOUT_MS = 5000;
+
+/**
+ * If no message has been received from the server in this window, treat
+ * the connection as stale and force-close. The server's heartbeat (30s
+ * per [[processor-ws-contract]]) means a healthy connection sees a
+ * message at least that often.
+ */
+export const STALE_CONNECTION_MS = 60000;

src/live/index.ts

@@ -0,0 +1,20 @@
+export { LiveBootstrap, getLiveClient } from './bootstrap';
+export { createCoalescer, type Coalescer } from './coalescer';
+export { useConnectionStore, type ConnectionStatus } from './connection-store';
+export { usePositionStore } from './position-store';
+export {
+  PositionEntrySchema,
+  InboundMessageSchema,
+  type PositionEntry,
+  type InboundMessage,
+  type OutboundMessage,
+  type SubscribeResult,
+} from './protocol';
+export { createLiveClient, type LiveClient, type PositionWithTopic } from './ws-client';
+export {
+  MAX_TRAIL_LENGTH,
+  RECONNECT_BACKOFF_MS,
+  RECONNECT_CEILING_MS,
+  STALE_CONNECTION_MS,
+  SUBSCRIBE_TIMEOUT_MS,
+} from './constants';

src/live/position-store.ts

@@ -0,0 +1,97 @@
+import { create } from 'zustand';
+import { MAX_TRAIL_LENGTH } from './constants';
+import type { PositionEntry } from './protocol';
+
+type PositionState = {
+  /** Most recent position per device, keyed by deviceId. */
+  latestByDevice: Map<string, PositionEntry>;
+  /** Bounded ring buffer of the last N positions per device. */
+  trailsByDevice: Map<string, PositionEntry[]>;
+  /** The device the operator selected for follow / detail. `null` = no selection. */
+  selectedDeviceId: string | null;
+  /** Which event's positions are currently in the store. `null` = none subscribed. */
+  activeEventId: string | null;
+};
+
+type PositionActions = {
+  /**
+   * Replace all device-keyed state with the snapshot from a new event
+   * subscription. Wipes the previous event's positions and trails;
+   * seeds each device's trail with its single snapshot position.
+   */
+  applySnapshot: (eventId: string, entries: PositionEntry[]) => void;
+
+  /**
+   * Apply a coalesced batch of positions. Latest-position-per-device
+   * gets a new object reference (selectors fire); positions that didn't
+   * change keep their old reference (selectors don't fire).
+   */
+  applyPositions: (entries: PositionEntry[]) => void;
+
+  /** Clear all device-keyed state. Used on event-switch and on logout. */
+  clearForEvent: () => void;
+
+  /** Select a device for follow / detail. Pass `null` to deselect. */
+  selectDevice: (deviceId: string | null) => void;
+};
+
+type Store = PositionState & PositionActions;
+
+const EMPTY_LATEST: Map<string, PositionEntry> = new Map();
+const EMPTY_TRAILS: Map<string, PositionEntry[]> = new Map();
+
+export const usePositionStore = create<Store>((set) => ({
+  latestByDevice: EMPTY_LATEST,
+  trailsByDevice: EMPTY_TRAILS,
+  selectedDeviceId: null,
+  activeEventId: null,
+
+  applySnapshot: (eventId, entries) => {
+    const latest = new Map<string, PositionEntry>();
+    const trails = new Map<string, PositionEntry[]>();
+    for (const e of entries) {
+      latest.set(e.deviceId, e);
+      trails.set(e.deviceId, [e]);
+    }
+    set({
+      activeEventId: eventId,
+      latestByDevice: latest,
+      trailsByDevice: trails,
+      selectedDeviceId: null,
+    });
+  },
+
+  applyPositions: (entries) => {
+    if (entries.length === 0) return;
+    set((state) => {
+      const latest = new Map(state.latestByDevice);
+      const trails = new Map(state.trailsByDevice);
+      for (const e of entries) {
+        latest.set(e.deviceId, e);
+        const tail = trails.get(e.deviceId) ?? [];
+        const last = tail[tail.length - 1];
+        // Skip duplicates from no-movement reports — same lat/lon as the
+        // last entry doesn't add a new trail point.
+        if (!last || last.lat !== e.lat || last.lon !== e.lon) {
+          const next = [...tail, e];
+          if (next.length > MAX_TRAIL_LENGTH) next.shift();
+          trails.set(e.deviceId, next);
+        }
+      }
+      return { latestByDevice: latest, trailsByDevice: trails };
+    });
+  },
+
+  clearForEvent: () => {
+    set({
+      activeEventId: null,
+      latestByDevice: EMPTY_LATEST,
+      trailsByDevice: EMPTY_TRAILS,
+      selectedDeviceId: null,
+    });
+  },
+
+  selectDevice: (deviceId) => {
+    set({ selectedDeviceId: deviceId });
+  },
+}));

src/live/protocol.ts

@@ -0,0 +1,75 @@
+import { z } from 'zod';
+
+/**
+ * Wire-format types matching `docs/wiki/synthesis/processor-ws-contract.md`.
+ * The producer sends JSON-encoded messages over WS; the SPA validates with
+ * zod and routes by `type`.
+ */
+
+export const PositionEntrySchema = z.object({
+  deviceId: z.string(),
+  lat: z.number(),
+  lon: z.number(),
+  ts: z.number(), // epoch ms
+  speed: z.number().optional(),
+  course: z.number().optional(),
+  accuracy: z.number().optional(),
+  attributes: z.record(z.string(), z.unknown()).optional(),
+});
+export type PositionEntry = z.infer<typeof PositionEntrySchema>;
+
+// ---------- Inbound ------------------------------------------------------
+
+const SubscribedMessage = z.object({
+  type: z.literal('subscribed'),
+  topic: z.string(),
+  id: z.string().optional(),
+  snapshot: z.array(PositionEntrySchema).default([]),
+});
+
+const UnsubscribedMessage = z.object({
+  type: z.literal('unsubscribed'),
+  topic: z.string(),
+  id: z.string().optional(),
+});
+
+const PositionMessage = z.object({
+  type: z.literal('position'),
+  topic: z.string(),
+  deviceId: z.string(),
+  lat: z.number(),
+  lon: z.number(),
+  ts: z.number(),
+  speed: z.number().optional(),
+  course: z.number().optional(),
+  accuracy: z.number().optional(),
+  attributes: z.record(z.string(), z.unknown()).optional(),
+});
+
+const ErrorMessage = z.object({
+  type: z.literal('error'),
+  topic: z.string().optional(),
+  id: z.string().optional(),
+  code: z.string(),
+  message: z.string().optional(),
+});
+
+export const InboundMessageSchema = z.discriminatedUnion('type', [
+  SubscribedMessage,
+  UnsubscribedMessage,
+  PositionMessage,
+  ErrorMessage,
+]);
+export type InboundMessage = z.infer<typeof InboundMessageSchema>;
+
+// ---------- Outbound -----------------------------------------------------
+
+export type OutboundMessage =
+  | { type: 'subscribe'; topic: string; id?: string }
+  | { type: 'unsubscribe'; topic: string; id?: string };
+
+// ---------- Subscribe result --------------------------------------------
+
+export type SubscribeResult =
+  | { ok: true; snapshot: PositionEntry[] }
+  | { ok: false; code: string; message?: string };

src/live/ws-client.ts

@@ -0,0 +1,281 @@
+import {
+  RECONNECT_BACKOFF_MS,
+  RECONNECT_CEILING_MS,
+  STALE_CONNECTION_MS,
+  SUBSCRIBE_TIMEOUT_MS,
+} from './constants';
+import {
+  InboundMessageSchema,
+  type OutboundMessage,
+  type PositionEntry,
+  type SubscribeResult,
+} from './protocol';
+import { useConnectionStore } from './connection-store';
+
+export type PositionWithTopic = PositionEntry & { topic: string };
+
+export interface LiveClient {
+  /** Open the WS. Idempotent. */
+  connect: () => void;
+  /** Permanent close. Won't reconnect. */
+  close: () => void;
+  /** Subscribe to a topic. Resolves on `subscribed` (with snapshot) or `error`. */
+  subscribe: (topic: string) => Promise<SubscribeResult>;
+  /** Unsubscribe from a topic. Best-effort; resolves immediately if not connected. */
+  unsubscribe: (topic: string) => Promise<void>;
+  /** Register a handler for `position` messages. Returns an unregister fn. */
+  onPosition: (handler: (msg: PositionWithTopic) => void) => () => void;
+}
+
+type ClientState =
+  | { kind: 'idle' }
+  | { kind: 'connecting'; ws: WebSocket }
+  | { kind: 'connected'; ws: WebSocket }
+  | { kind: 'reconnecting'; attempt: number; timer: ReturnType<typeof setTimeout> }
+  | { kind: 'closed' };
+
+type PendingSubscribe = {
+  resolve: (result: SubscribeResult) => void;
+  timer: ReturnType<typeof setTimeout>;
+};
+
+let _idSeq = 0;
+function nextCorrelationId(): string {
+  _idSeq = (_idSeq + 1) % Number.MAX_SAFE_INTEGER;
+  return `c${_idSeq}`;
+}
+
+/**
+ * Resolve a (possibly relative) WS URL against the current page origin.
+ *
+ * - `'/ws-live'` → `'ws://localhost:5173/ws-live'` (dev) or `'wss://stage.trmtracking.org/ws-live'` (stage).
+ * - `'wss://elsewhere.example.com/...'` passes through unchanged.
+ *
+ * The browser's `WebSocket` constructor *requires* `ws(s)://` schemes
+ * and an absolute URL; same-origin path strings have to be resolved
+ * client-side.
+ */
+function toAbsoluteWsUrl(path: string): string {
+  if (/^wss?:\/\//.test(path)) return path;
+  const protocol = window.location.protocol === 'https:' ? 'wss:' : 'ws:';
+  const host = window.location.host;
+  return `${protocol}//${host}${path.startsWith('/') ? path : `/${path}`}`;
+}
+
+export function createLiveClient(opts: { url: string }): LiveClient {
+  let state: ClientState = { kind: 'idle' };
+  /** Topics the user has expressed interest in, replayed on every reconnect. */
+  const subscriptions = new Set<string>();
+  /** In-flight subscribe / unsubscribe requests waiting for a server reply. */
+  const pending = new Map<string, PendingSubscribe>();
+  /** Position handlers — there's only one in practice (the coalescer's push) but keep extensible. */
+  const positionHandlers = new Set<(msg: PositionWithTopic) => void>();
+  /** Last time we received any message; if it goes stale, force-reconnect. */
+  let lastMessageAt = 0;
+  let staleCheckTimer: ReturnType<typeof setInterval> | null = null;
+
+  function sendRaw(msg: OutboundMessage): void {
+    if (state.kind !== 'connected') return;
+    state.ws.send(JSON.stringify(msg));
+  }
+
+  function startStaleCheck(): void {
+    if (staleCheckTimer) clearInterval(staleCheckTimer);
+    staleCheckTimer = setInterval(() => {
+      if (state.kind !== 'connected') return;
+      if (Date.now() - lastMessageAt > STALE_CONNECTION_MS) {
+        // Server's heartbeat hasn't fired; treat as a dead connection.
+        state.ws.close(4000, 'stale');
+      }
+    }, STALE_CONNECTION_MS / 2);
+  }
+
+  function stopStaleCheck(): void {
+    if (staleCheckTimer) {
+      clearInterval(staleCheckTimer);
+      staleCheckTimer = null;
+    }
+  }
+
+  function onMessage(raw: unknown): void {
+    lastMessageAt = Date.now();
+    let parsed;
+    try {
+      parsed = InboundMessageSchema.safeParse(JSON.parse(String(raw)));
+    } catch {
+      return; // malformed JSON; drop
+    }
+    if (!parsed.success) return;
+    const msg = parsed.data;
+
+    switch (msg.type) {
+      case 'subscribed': {
+        if (msg.id) {
+          const p = pending.get(msg.id);
+          if (p) {
+            clearTimeout(p.timer);
+            pending.delete(msg.id);
+            p.resolve({ ok: true, snapshot: msg.snapshot });
+          }
+        }
+        break;
+      }
+      case 'unsubscribed': {
+        if (msg.id) {
+          const p = pending.get(msg.id);
+          if (p) {
+            clearTimeout(p.timer);
+            pending.delete(msg.id);
+            p.resolve({ ok: true, snapshot: [] });
+          }
+        }
+        break;
+      }
+      case 'position': {
+        const positionMsg: PositionWithTopic = {
+          deviceId: msg.deviceId,
+          lat: msg.lat,
+          lon: msg.lon,
+          ts: msg.ts,
+          speed: msg.speed,
+          course: msg.course,
+          accuracy: msg.accuracy,
+          attributes: msg.attributes,
+          topic: msg.topic,
+        };
+        for (const h of positionHandlers) h(positionMsg);
+        break;
+      }
+      case 'error': {
+        if (msg.id) {
+          const p = pending.get(msg.id);
+          if (p) {
+            clearTimeout(p.timer);
+            pending.delete(msg.id);
+            p.resolve({ ok: false, code: msg.code, message: msg.message });
+          }
+        }
+        useConnectionStore.getState().setStatus(useConnectionStore.getState().status, {
+          error: msg.message ?? msg.code,
+        });
+        break;
+      }
+    }
+  }
+
+  function connect(): void {
+    if (state.kind === 'connecting' || state.kind === 'connected') return;
+    if (state.kind === 'closed') return;
+
+    useConnectionStore.getState().setStatus('connecting', { error: null });
+
+    let ws: WebSocket;
+    try {
+      ws = new WebSocket(toAbsoluteWsUrl(opts.url));
+    } catch (err) {
+      useConnectionStore.getState().setStatus('reconnecting', {
+        attempt: state.kind === 'reconnecting' ? state.attempt + 1 : 1,
+        error: err instanceof Error ? err.message : 'WebSocket constructor failed',
+      });
+      scheduleReconnect();
+      return;
+    }
+    state = { kind: 'connecting', ws };
+
+    ws.addEventListener('open', () => {
+      state = { kind: 'connected', ws };
+      lastMessageAt = Date.now();
+      useConnectionStore.getState().setStatus('connected', { attempt: 0 });
+      startStaleCheck();
+      // Replay every active subscription. New correlation IDs because the
+      // old ones (from before the disconnect) are stale.
+      for (const topic of subscriptions) {
+        sendRaw({ type: 'subscribe', topic });
+      }
+    });
+
+    ws.addEventListener('message', (ev) => onMessage(ev.data));
+
+    ws.addEventListener('close', () => {
+      stopStaleCheck();
+      // Reject all pending subscribes so callers don't hang.
+      for (const [id, p] of pending) {
+        clearTimeout(p.timer);
+        p.resolve({ ok: false, code: 'disconnected' });
+        pending.delete(id);
+      }
+      if (state.kind === 'closed') return;
+      const attempt = state.kind === 'reconnecting' ? state.attempt + 1 : 1;
+      useConnectionStore.getState().setStatus('reconnecting', { attempt });
+      scheduleReconnect(attempt);
+    });
+
+    ws.addEventListener('error', () => {
+      // The 'close' event fires next; reconnect is scheduled there.
+    });
+  }
+
+  function scheduleReconnect(attempt = 1): void {
+    if (state.kind === 'closed') return;
+    const delay = RECONNECT_BACKOFF_MS[attempt - 1] ?? RECONNECT_CEILING_MS;
+    const cappedDelay = Math.min(delay, RECONNECT_CEILING_MS);
+    const timer = setTimeout(() => {
+      if (state.kind === 'closed') return;
+      connect();
+    }, cappedDelay);
+    state = { kind: 'reconnecting', attempt, timer };
+  }
+
+  function close(): void {
+    if (state.kind === 'connecting' || state.kind === 'connected') {
+      state.ws.close(1000, 'client closed');
+    }
+    if (state.kind === 'reconnecting') {
+      clearTimeout(state.timer);
+    }
+    stopStaleCheck();
+    for (const [, p] of pending) {
+      clearTimeout(p.timer);
+      p.resolve({ ok: false, code: 'closed' });
+    }
+    pending.clear();
+    subscriptions.clear();
+    state = { kind: 'closed' };
+    useConnectionStore.getState().setStatus('disconnected', { attempt: 0 });
+  }
+
+  async function subscribe(topic: string): Promise<SubscribeResult> {
+    subscriptions.add(topic);
+    if (state.kind !== 'connected') {
+      // Topic is remembered; replayed on next 'open'. Caller should retry
+      // or wait for 'connected' status if they want the snapshot.
+      return { ok: false, code: 'not-connected' };
+    }
+    const id = nextCorrelationId();
+    return new Promise<SubscribeResult>((resolve) => {
+      const timer = setTimeout(() => {
+        pending.delete(id);
+        resolve({ ok: false, code: 'timeout' });
+      }, SUBSCRIBE_TIMEOUT_MS);
+      pending.set(id, { resolve, timer });
+      sendRaw({ type: 'subscribe', topic, id });
+    });
+  }
+
+  async function unsubscribe(topic: string): Promise<void> {
+    subscriptions.delete(topic);
+    if (state.kind !== 'connected') return;
+    const id = nextCorrelationId();
+    sendRaw({ type: 'unsubscribe', topic, id });
+    // Best-effort: don't wait for ack. The server cleans up regardless.
+  }
+
+  function onPosition(handler: (msg: PositionWithTopic) => void): () => void {
+    positionHandlers.add(handler);
+    return () => {
+      positionHandlers.delete(handler);
+    };
+  }
+
+  return { connect, close, subscribe, unsubscribe, onPosition };
+}

src/main.tsx

@@ -4,6 +4,7 @@ import './styles/globals.css';
 import App from './App.tsx';
 import { RuntimeConfigProvider } from '@/config/provider';
 import { AuthBootstrap } from '@/auth';
+import { LiveBootstrap } from '@/live';
 import { preloadSprites } from '@/map/core/sprite-preload';
 
 // Fire-and-forget preload of the map sprite registry. The promise is
@@ -15,7 +16,9 @@ createRoot(document.getElementById('root')!).render(
   <StrictMode>
     <RuntimeConfigProvider>
       <AuthBootstrap>
+        <LiveBootstrap>
           <App />
+        </LiveBootstrap>
       </AuthBootstrap>
     </RuntimeConfigProvider>
   </StrictMode>,