feat: task 2.1 MapView singleton + mapReady gate

- pnpm add maplibre-gl + -D @types/geojson.
- src/map/core/styles.ts: defaultStyle (OSM raster bootstrap; 2.2
  replaces with the basemap-switcher descriptor table).
- src/map/core/map-view.tsx: module-level Map singleton lazily created
  on first <MapView> mount, attached to a class="trm-map-host" detached
  <div> that React refs append/remove on mount/unmount. Style-data
  lifecycle flips mapReady false on every styledata event, polls
  loaded() at 33ms intervals, flips ready true once the style is
  loaded — the canonical MapLibre style-swap dance.
- Exports getMap()/getMapReady()/subscribeMapReady()/useMapReady (via
  useSyncExternalStore for SSR-safe + concurrent-safe reads). getMap()
  throws if called pre-mount; the explicit failure mode beats a
  null-able top-level export.
- src/routes/_authed/monitor.tsx: new /monitor route, full-viewport
  <MapView /> for 2.1 (no children — subsequent tasks plug in here).
- src/routes/_authed/index.tsx: home-page card now links to /monitor.
- eslint.config.js: override for src/map/** + src/live/** disables
  react-refresh/only-export-components. Same pattern as the existing
  overrides for shadcn primitives and route files.

Deviation: spec sketched a top-level `map` constant export; implemented
as `getMap(): MapLibreMap` (a function) so the singleton stays lazy
until <MapView> mounts. Top-level constant would either force eager
init (breaks SSR/tests) or be nullable (footgun). The function form
throws a clear error if called pre-mount.

Bundle: /monitor lazy chunk is 1MB raw / 274KB gzipped (MapLibre + CSS).
Other routes unaffected. Vite chunk-size warning is harmless.

src/routes/_authed/index.tsx

@@ -1,5 +1,5 @@
 import { useEffect } from 'react';
-import { createFileRoute, useNavigate } from '@tanstack/react-router';
+import { createFileRoute, Link, useNavigate } from '@tanstack/react-router';
 import { useAuthStore } from '@/auth';
 import { LogoutButton } from '@/ui/components/logout-button';
 import { Card, CardContent, CardHeader, CardTitle } from '@/ui/primitives/card';
@@ -40,11 +40,16 @@ function HomePage() {
         <CardHeader>
           <CardTitle>Live monitoring map</CardTitle>
         </CardHeader>
-        <CardContent>
+        <CardContent className="space-y-3">
           <p className="text-sm text-muted-foreground">
-            The live-position map lands in Phase 2 once the Processor's WebSocket endpoint is
-            shipped (Phase 1.5 — already complete on the processor side).
+            Watch registered devices report their positions in real time.
           </p>
+          <Link
+            to="/monitor"
+            className="inline-flex items-center gap-1 text-sm font-medium underline underline-offset-4"
+          >
+            Open live map →
+          </Link>
         </CardContent>
       </Card>
     </div>

src/routes/_authed/monitor.tsx

@@ -0,0 +1,17 @@
+import { createFileRoute } from '@tanstack/react-router';
+import { MapView } from '@/map/core/map-view';
+
+export const Route = createFileRoute('/_authed/monitor')({
+  component: MonitorPage,
+});
+
+function MonitorPage() {
+  return (
+    // 3.5rem ≈ 56 px reserved for a future top-bar (Phase 3 chrome).
+    // For now the parent _authed layout has no top-bar, so the map can
+    // fill the full viewport — adjust when chrome lands.
+    <div className="relative h-[calc(100vh-0rem)] w-full">
+      <MapView />
+    </div>
+  );
+}