fix(handtracking): absorb React StrictMode double-mount

In dev, <StrictMode> intentionally mounts → unmounts → remounts each
effect to surface non-idempotent code. The hand tracking hooks were
calling getUserMedia and creating MediaPipe / WebSocket runtimes on
every mount, which in practice ran the full start/stop/start cycle
inside a few milliseconds and pushed WebGL over its limit on top
of the loaded scene → context lost.

Add HAND_TRACKING_RUNTIME_START_DELAY_MS (80ms) and delay the actual
start() call behind a setTimeout in both useBrowserHandTracking and
useRemoteHandTracking. The cleanup clears the timer, so a fast
mount/unmount never reaches start(). 80ms is invisible to the user
(<5 frames at 60fps) and also absorbs rapid `nearby` toggles at
trigger borders.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/data/handTrackingConfig.ts

@@ -9,3 +9,8 @@ export const HAND_TRACKING_BROWSER_WASM_URL =
 export const HAND_TRACKING_BROWSER_MODEL_URL =
   "https://storage.googleapis.com/mediapipe-models/hand_landmarker/hand_landmarker/float16/1/hand_landmarker.task";
 export const HAND_TRACKING_BROWSER_DELEGATE: "CPU" | "GPU" = "CPU";
+
+// Delay before the runtime actually starts after `enabled` flips to true.
+// Absorbs React StrictMode's mount/unmount/mount cycle in dev and rapid
+// `nearby` toggles at trigger borders. Invisible to the user (~5 frames).
+export const HAND_TRACKING_RUNTIME_START_DELAY_MS = 80;