feat: global master clock for drift-free multi-sequencer timing

Replace independent Tone.Clock per sequencer/pianoroll with a single
shared master clock running at 960 Hz in audioEngine.

Architecture:
- Master clock starts/stops with audio engine (startAudio/stopAudio)
- Widgets subscribe via subscribeTick(id, callback) receiving
  (audioTime, elapsed) on every tick
- Each widget derives its own step/position from elapsed time and
  its own BPM, so different BPMs stay perfectly in sync
- BPM/steps/bars changes are read from refs (no clock restart needed)

Benefits:
- All timing derived from one clock source = zero relative drift
- No clock recreation on param changes = no glitches
- 960 Hz tick rate ≈ 1ms precision (plenty for musical timing)
- Sequencer at 80 BPM and 160 BPM maintain perfect 1:2 ratio forever

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/engine/audioEngine.js

@@ -12,6 +12,48 @@ const audioNodes = {};
 // Active keyboard state
 const keyboardState = { frequency: 440, gate: false };
 
+// ==================== Global Master Clock ====================
+// Single high-resolution clock (960 ticks/sec ≈ 1ms precision).
+// All sequencers/piano rolls derive their timing from this.
+const MASTER_TICK_RATE = 960; // Hz
+let _masterClock = null;
+let _masterTime = 0; // audio-context seconds at clock start
+const _tickListeners = new Map(); // id → callback(audioTime, elapsed)
+
+export function getMasterTime() {
+  if (!_masterClock) return 0;
+  return Tone.now() - _masterTime;
+}
+
+export function subscribeTick(id, callback) {
+  _tickListeners.set(id, callback);
+}
+
+export function unsubscribeTick(id) {
+  _tickListeners.delete(id);
+}
+
+function startMasterClock() {
+  if (_masterClock) return;
+  _masterTime = Tone.now();
+  _masterClock = new Tone.Clock((time) => {
+    const elapsed = time - _masterTime;
+    for (const cb of _tickListeners.values()) {
+      cb(time, elapsed);
+    }
+  }, MASTER_TICK_RATE);
+  _masterClock.start();
+}
+
+function stopMasterClock() {
+  if (_masterClock) {
+    try { _masterClock.stop(); } catch {}
+    try { _masterClock.dispose(); } catch {}
+    _masterClock = null;
+  }
+  _tickListeners.clear();
+}
+
 // ==================== Node creation ====================
 
 function createNode(mod) {
@@ -434,18 +476,21 @@ export function triggerKeyboard(moduleId, freq, gate) {
 export async function startAudio() {
   await Tone.start();
   state.isRunning = true;
+  startMasterClock();
 
   // Rebuild entire audio graph
   rebuildGraph();
 }
 
 export function stopAudio() {
-  // Stop and reset Transport so pianoroll/sequencer Parts don't get stranded
+  stopMasterClock();
+
+  // Stop and reset Transport
   try {
     Tone.getTransport().stop();
-    Tone.getTransport().cancel(); // Remove all scheduled events
+    Tone.getTransport().cancel();
     Tone.getTransport().position = 0;
-  } catch (e) { /* ignore if Transport not started */ }
+  } catch (e) {}
 
   // Destroy all nodes
   for (const id of Object.keys(audioNodes)) {