feat: wire the dev Motion button (label reflects state); chip toggle routes through the same setter

public/js/exhibit.js

@@ -43,8 +43,16 @@ let camMode = 'none';
 
 /* Diagnostic: freeze all ghost motion (bob/sway/wander/path) so ghosts sit at their
  * static spawn position — tells tracking jitter apart from the float animation.
- * Defaults ON in dev; tap the tracking HUD chip to toggle. */
+ * Defaults ON in dev; toggle with the "Motion" button (or the tracking chip). */
 let freezeGhosts = false;
+function setFreeze(v) {
+  freezeGhosts = v;
+  const b = $('#motionBtn');
+  if (b) {
+    b.textContent = `Motion: ${v ? 'frozen' : 'on'}`;
+    b.classList.toggle('frozen', v);
+  }
+}
 
 let info = { mode: 'dev' };
 let scene3, camera3, renderer, video, canvas2d, ctx2d;
@@ -233,7 +241,13 @@ async function start() {
 
   // dev-mode pose readout + tap-to-cycle modes
   if (info.mode === 'dev') {
-    freezeGhosts = true;   // start static so tracking jitter is isolated from ghost float
+    // start static so tracking jitter can't be mistaken for the ghost's own float
+    const mb = $('#motionBtn');
+    if (mb) {
+      mb.classList.remove('hidden');
+      mb.addEventListener('click', () => setFreeze(!freezeGhosts));
+    }
+    setFreeze(true);
     dbgEl = $('#dbg');
     if (dbgEl) {
       dbgEl.style.display = 'block';
@@ -267,7 +281,7 @@ async function start() {
     const trk = $('#trk');
     if (trk) {
       trk.style.pointerEvents = 'auto';
-      trk.addEventListener('click', () => { freezeGhosts = !freezeGhosts; });
+      trk.addEventListener('click', () => setFreeze(!freezeGhosts));
     }
   }