perf: remove throttled-trace residuals — program churn, per-frame rect, HUD clock

A 4x-CPU-throttled retrace (the honest weak-hardware proxy) surfaced
three residual per-frame costs; stack attribution pinned each:

- getParameters/getProgramCacheKey (~4% of main thread): every pooled
  label sprite map swap set material.needsUpdate, bumping
  material.version and forcing full program re-resolution next render.
  Swapping between two non-null cached textures never changes the
  compiled program (USE_MAP define unchanged) — new _setLabelMap()
  helper only flags needsUpdate on a null<->texture transition, used at
  all 7 swap sites.
- getBoundingClientRect (~1.2%): the 3D highway's per-frame canvas-size
  self-check forced a layout read every frame. The CSS-box drift read
  now runs every 10th frame (or when the wrap isn't pinned); the
  backing-store comparison stays per-frame with cheap property reads
  and forces an immediate box read + applySize when it fires.
- set textContent: the core 60 Hz HUD clock rewrote hud-time (and
  getElementById'd it) every tick for a display that changes 1/s — now
  write-on-change with a cached element ref.

(The remaining textContent writer in the trace is notedetect's
badges.js — external repo, to be filed there.)

tests/js: resize-reframe shape test updated for the hoisted _bsChanged
gate, incl. an assertion that the throttle can never delay the
backing-store path.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

static/app.js

@@ -9777,6 +9777,12 @@ async function startSongCountIn() {
 
 // Time display + highway sync
 let lastAudioTime = 0;
+// hud-time write cache: the 60 Hz tick below used to rewrite textContent
+// (and getElementById) every tick even though the mm:ss display only
+// changes once a second — each write invalidates layout. Write-on-change
+// with a cached element ref (re-resolved if detached).
+let _hudTimeEl = null;
+let _hudTimeLast = '';
 setInterval(() => {
     let ct = _audioTime();
     const dur = _audioDuration();
@@ -9804,7 +9810,12 @@ setInterval(() => {
             ct = lastAudioTime;
         }
         lastAudioTime = ct;
-        document.getElementById('hud-time').textContent = `${formatTime(ct)} / ${formatTime(dur)}`;
+        const hudText = `${formatTime(ct)} / ${formatTime(dur)}`;
+        if (hudText !== _hudTimeLast) {
+            if (!_hudTimeEl || !_hudTimeEl.isConnected) _hudTimeEl = document.getElementById('hud-time');
+            if (_hudTimeEl) _hudTimeEl.textContent = hudText;
+            _hudTimeLast = hudText;
+        }
         if (dur) {
             _maybeRefreshSectionPracticeDuration(dur);
         }