fix(handtracking): stabilize provider root and linger enabled

(1) HandTrackingProvider always renders the same JSX root
(HandTrackingRuntime) so toggling `enabled` no longer remounts the
<Canvas> below — that remount was destroying the WebGL context every
time the player entered an interaction zone.

(2) Add HAND_TRACKING_LINGER_MS (2s) cooldown on `enabled` so brief
walk-throughs of a trigger zone don't tear down MediaPipe before it
has time to initialize the webcam + model + first frame (cold start
~800ms).

Resolves the WebGL context lost + respawn loop and restores visible
hand tracking in the backend runtime. Browser JS runtime detection
quality is a separate follow-up.

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

src/data/handTrackingConfig.ts

@@ -14,3 +14,9 @@ export const HAND_TRACKING_BROWSER_DELEGATE: "CPU" | "GPU" = "CPU";
 // 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;
+
+// How long the hand tracking stays active after the trigger condition
+// (nearby / holding / repair step) turns off. Gives MediaPipe enough time
+// to initialize webcam + model + first frame inference before we cleanup,
+// so the user actually sees their hands when entering a zone briefly.
+export const HAND_TRACKING_LINGER_MS = 2000;

src/providers/gameplay/HandTrackingProvider.tsx

@@ -1,4 +1,6 @@
 import type { ReactNode } from "react";
+import { useEffect, useState } from "react";
+import { HAND_TRACKING_LINGER_MS } from "@/data/handTrackingConfig";
 import { useSceneMode } from "@/hooks/debug/useSceneMode";
 import { useDebugStore } from "@/hooks/debug/useDebugStore";
 import { useInteraction } from "@/hooks/interaction/useInteraction";
@@ -38,37 +40,74 @@ export function HandTrackingProvider({
     }
   });
   const { nearby, holding, handHolding } = useInteraction();
-  const enabled =
+  const requested =
     repairNeedsHands ||
     (sceneMode === "physics" && (nearby || holding || handHolding));
 
-  if (!enabled) {
-    return (
-      <HandTrackingContext value={HAND_TRACKING_IDLE_SNAPSHOT}>
-        {children}
-      </HandTrackingContext>
-    );
-  }
+  // Keep the runtime active a little after `requested` turns off so
+  // MediaPipe has time to initialize the webcam + model + first frame
+  // before being torn down. Without this, a quick walk-through of a
+  // trigger zone never produces a detected hand and the user sees
+  // nothing.
+  const enabled = useLingeredFlag(requested, HAND_TRACKING_LINGER_MS);
 
-  return <ActiveHandTrackingProvider>{children}</ActiveHandTrackingProvider>;
+  // Always render the same JSX root (HandTrackingRuntime). Returning
+  // different element types from this provider would force React to
+  // remount its entire subtree — including the <Canvas> below — every
+  // time `enabled` toggles, which destroys the WebGL context.
+  return (
+    <HandTrackingRuntime enabled={enabled}>{children}</HandTrackingRuntime>
+  );
 }
 
-function ActiveHandTrackingProvider({
+function useLingeredFlag(value: boolean, lingerMs: number): boolean {
+  const [latched, setLatched] = useState(value);
+
+  // Asymmetric sync: snap up immediately when `value` becomes true,
+  // debounce the down transition by `lingerMs`. The setLatched(true)
+  // call below is intentionally a direct setState inside an effect
+  // because that is exactly the pattern we want (mirror upward edge,
+  // delay downward edge), and there is no equivalent without it.
+  useEffect(() => {
+    if (value) {
+      // eslint-disable-next-line react-hooks/set-state-in-effect -- intentional upward edge sync, see hook comment
+      setLatched(true);
+      return undefined;
+    }
+
+    const timer = window.setTimeout(() => {
+      setLatched(false);
+    }, lingerMs);
+
+    return () => {
+      window.clearTimeout(timer);
+    };
+  }, [value, lingerMs]);
+
+  return latched;
+}
+
+function HandTrackingRuntime({
+  enabled,
   children,
 }: {
+  enabled: boolean;
   children: ReactNode;
 }): React.JSX.Element {
   const handTrackingSource = useDebugStore((debug) =>
     debug.getHandTrackingSource(),
   );
   const backendSnapshot = useRemoteHandTracking({
-    enabled: handTrackingSource === "backend",
+    enabled: enabled && handTrackingSource === "backend",
   });
   const browserSnapshot = useBrowserHandTracking({
-    enabled: handTrackingSource === "browser",
+    enabled: enabled && handTrackingSource === "browser",
   });
-  const snapshot =
-    handTrackingSource === "browser" ? browserSnapshot : backendSnapshot;
+  const snapshot = !enabled
+    ? HAND_TRACKING_IDLE_SNAPSHOT
+    : handTrackingSource === "browser"
+      ? browserSnapshot
+      : backendSnapshot;
 
   return <HandTrackingContext value={snapshot}>{children}</HandTrackingContext>;
 }