feat(booth): blink the Entry/Exit lights on radar presence (mirror relay 3)

The on-screen Hyrje/Dalje barrier lights were 2-state (green=free / red=busy)
off the camera lane-status only — they couldn't show the radar-only "detected,
not yet confirmed" state that makes the physical button lamp (relay 3) blink.
Now they mirror the lamp's 3-state rule per lane:
  radar present + camera not busy → BLINK green↔red (~1 Hz)
  camera busy                     → SOLID red
  otherwise                       → SOLID green

End-to-end:
- LanePresence (lane-presence.ts): subscribes to deviceEvents.onInput, resolves
  each presence edge to its lane via the new direction-agnostic presenceLaneOf()
  (device-resolve.ts) — entry AND exit, unlike the entry-gated relayForPresence
  the one-car-one-ticket gate uses — and emits a lane-presence {entry,exit} bus
  event on change. Wired in server.ts (start + onClose).
- WS forwards it (hello snapshot + push) into live-store.radar.
- BarrierLight (BoothScreen.tsx) is now 3-state; blinks via the .lane-blink
  keyframe (index.css), which holds solid-red under prefers-reduced-motion.

Same input + same rule as the lamp, so the screen and the post never disagree.

A new test (lane-presence.test.ts) caught a real bug: the first cut reused
relayForPresence, so the EXIT lane never resolved (it's entry-gated) and never
blinked — presenceLaneOf fixes it. Covers entry/exit independence, de-dupe
across several radars on one lane, and ignoring non-presence inputs.

Full workspace build/lint/test green (185 server tests). Updated the
button-light-indicator wiki page ("On-screen twin").

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V

apps/server/src/device-events.ts

@@ -86,6 +86,16 @@ export interface LaneStatusEvent {
   readonly exit: boolean; // true = busy (a vehicle is at the exit vicinity)
 }
 
+/** Per-lane RADAR presence — a vehicle-presence INPUT (loop/radar) is shorted at the
+ *  entry/exit barrier, i.e. "something is in the lane vicinity" BEFORE the camera has
+ *  confirmed a vehicle. Same signal that makes the physical button lamp (relay 3) blink:
+ *  radar-present + camera-not-busy. Drives the booth's barrier light blink. Advisory only —
+ *  it gates nothing. See wiki/concepts/button-light-indicator.md. */
+export interface LanePresenceEvent {
+  readonly entry: boolean; // true = a presence input on an entry barrier is active
+  readonly exit: boolean; // true = a presence input on an exit barrier is active
+}
+
 class DeviceEventBus extends EventEmitter {
   emitInput(event: DeviceInputEvent): void {
     this.emit("input", event);
@@ -148,6 +158,16 @@ class DeviceEventBus extends EventEmitter {
     this.on("lane-status", cb);
     return () => this.off("lane-status", cb);
   }
+
+  /** Emitted whenever a lane's RADAR presence CHANGES (a presence input shorted/cleared
+   *  at an entry/exit barrier). Drives the booth barrier light's blink. Advisory only. */
+  emitLanePresence(event: LanePresenceEvent): void {
+    this.emit("lane-presence", event);
+  }
+  onLanePresence(cb: (event: LanePresenceEvent) => void): () => void {
+    this.on("lane-presence", cb);
+    return () => this.off("lane-presence", cb);
+  }
 }
 
 /** Process-wide device event bus. */