fix(sim): cancel in-flight volume fades so audio isn't silenced after landing

Operator: audio turned off sometimes after landing on a new altitude. Root cause:
fadeVolume started a setInterval ramp that was never cancelled. When restAudio faded
the idle element to 0 and — within FADE_MS — that same element became the newly-landed
ACTIVE scale (assignElements reuses an element per soundtrack url), the stale fade-to-0
kept running and dragged the now-active element back to silence. Fix: cancelFade()
supersedes any in-flight fade when a new fade starts (fadeVolume) or a direct volume
is set (ensurePlaying). E2E proves a new fade supersedes a stale one (fails without
the fix).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

simulator/static/app.js

@@ -1076,19 +1076,33 @@ aud.addEventListener("error", updateAudioStatus);
 audB.addEventListener("error", updateAudioStatus);
 setInterval(updateAudioStatus, 400);
 
+// Cancel any in-flight fade on an element. A fade is a setInterval ramping volume;
+// if a stale one keeps running after a new fade or a direct volume set, it drags the
+// element back toward its old target (e.g. a fade-to-0 on what becomes the ACTIVE
+// element silences it — the "audio off after landing" bug).
+function cancelFade(el) {
+  if (el._fadeIv) { clearInterval(el._fadeIv); el._fadeIv = null; }
+}
+
 function fadeVolume(el, to, ms, done) {
   // setInterval (not requestAnimationFrame): rAF throttles/pauses when the tab is
   // backgrounded, which would stall the gain ramp; a timer fires regardless.
+  cancelFade(el);                       // supersede any fade already running on this element
   const from = el.volume, steps = Math.max(1, Math.round(ms / 30));
   let i = 0;
   const iv = setInterval(() => {
     i += 1;
     const k = Math.min(1, i / steps);
     el.volume = Math.min(1, Math.max(0, from + (to - from) * k));
-    if (k >= 1) { clearInterval(iv); if (done) done(); }
+    if (k >= 1) { clearInterval(iv); el._fadeIv = null; if (done) done(); }
   }, 30);
+  el._fadeIv = iv;
 }
 
+// Diagnostic seam (sibling of window.__hefState): expose the fade so E2E can verify a
+// new fade supersedes a stale one rather than racing it.
+if (typeof window !== "undefined") window.__hefFade = fadeVolume;
+
 // Fallback per-scale soundtrack paths, used when the ring from /api/ring lacks the
 // `audio` field (e.g. a server process started before the audio field existed).
 // Mirrors simulator/build_pool_manifest.py SCALE_AUDIO — the 5 scales + files are
@@ -1128,6 +1142,7 @@ function ensurePlaying(el, url, vol) {
     if (pr) pr.then(() => { audLastErr = ""; updateAudioStatus(); })
             .catch((e) => { audLastErr = "play BLOCKED: " + ((e && e.name) || e); updateAudioStatus(); });
   }
+  cancelFade(el);                       // a direct volume set supersedes any in-flight fade on this element
   el.volume = HEFScrub.clamp01(vol);
 }