fix(#7): anchor the audio master clock so it never jumps backward (startup freeze)

Cold-start root cause: on a slow first load the <audio> element starts late, so
the audio-ready gate's wall-clock fallback has already advanced playback a second
or two by the time audio finally begins at currentTime≈0. The master clock then
snapped back toward 0, every buffered frame read as "in the future", and
renderFrame() deadlocked until audio caught up — the freeze. A refresh warms the
cache, audio starts immediately, the gap (and the freeze) vanish — which is why
reloading "fixes" it.

Fix: the first time audio is genuinely playing, capture the offset between the
wall clock and the audio clock and add it back, so the master clock follows
audio's *rate* without ever moving backward. When audio starts promptly the
offset is ~0, so normal playback is unchanged.

experiments/freeze_repro.js models a 2s-late audio start with a realistic jitter
buffer + 60fps render loop: the original code and a naive `currentTime > 0` guard
render 0-1 of ~96 frames after audio starts (frozen ~4s); the anchored clock
renders 96/96 smooth. Real-browser regression (muted Chrome): normal playback
unaffected at ~29 fps.

experiments/freeze_repro.js

@@ -0,0 +1,59 @@
+/*
+ * freeze_repro2.js — Model the COLD-START freeze (issue #7): audio starts late,
+ * so the wall-clock fallback renders ahead, then the master clock snaps back to
+ * audioEl.currentTime (~0) and playback freezes until audio catches up.
+ *
+ * Faithfully simulates: a server feeding frames at fps, a capped jitter buffer,
+ * a 60fps render loop, and the app.js master-clock + frame-gate logic — under
+ * three clock policies.
+ */
+function sim({ policy, audioDelayMs }) {
+  const fps = 24, rAF = 1000 / 60, DURATION = 6000;
+  const buf = [];
+  let lastServerFrame = -1, audioStart = null;
+  const renderTimes = [];
+
+  for (let t = 0; t <= DURATION; t += rAF) {
+    // server feeds frames in real time; jitter buffer caps at 20 (drops oldest)
+    const want = Math.floor(t / 1000 * fps);
+    while (lastServerFrame < want) buf.push({ time: ++lastServerFrame / fps });
+    while (buf.length > 20) buf.shift();
+
+    const audioPlaying = t >= audioDelayMs;
+    if (audioPlaying && audioStart === null) audioStart = t;
+    const audioCurrent = audioPlaying ? (t - audioStart) / 1000 : 0;
+    const wall = t / 1000;
+
+    // ── master clock policy ──
+    let master;
+    if (policy === 'broken')        master = audioPlaying ? audioCurrent : wall;
+    else if (policy === 'guard')    master = (audioPlaying && audioCurrent > 0) ? audioCurrent : wall;
+    else if (policy === 'monotonic') {
+      // fix: anchor audio so the clock never jumps backward
+      if (audioPlaying) {
+        if (sim._off === undefined) sim._off = wall - audioCurrent; // capture once
+        master = audioCurrent + sim._off;
+      } else master = wall;
+    }
+
+    // ── frame gate (verbatim from app.js) ──
+    if (buf.length) {
+      while (buf.length > 1 && buf[0].time < master - 0.1) buf.shift();
+      if (buf[0].time <= master + 0.05) { buf.shift(); renderTimes.push(t); }
+    }
+  }
+  delete sim._off;
+  const last = renderTimes[renderTimes.length - 1] ?? 0;
+  const afterAudio = renderTimes.filter(t => t > audioDelayMs).length;
+  const trailingStallMs = Math.round(DURATION - last);   // froze until the end?
+  return { rendered: renderTimes.length, afterAudio, trailingStallMs };
+}
+
+const EXPECTED_AFTER = Math.round((6000 - 2000) / 1000 * 24); // ~96 frames in the 2-6s window
+console.log(`audio starts 2s late (cold start). Expect ~${EXPECTED_AFTER} frames AFTER 2s if smooth:\n`);
+for (const policy of ['broken', 'guard', 'monotonic']) {
+  const r = sim({ policy, audioDelayMs: 2000 });
+  const frozen = r.afterAudio < 5;
+  console.log(`  ${policy.padEnd(10)} total ${String(r.rendered).padStart(3)}  | after-audio ${String(r.afterAudio).padStart(3)}/${EXPECTED_AFTER}  | `
+    + (frozen ? `FROZE (stuck ${r.trailingStallMs}ms to end)` : `smooth`));
+}