fix: capture master clock start time from first tick callback

The _masterTime was captured from Tone.now() BEFORE the clock started,
but the time parameter in Tone.Clock callbacks comes from a different
scheduler timeline. This caused elapsed to drift systematically.

Now _masterTime is set from the first callback's own time parameter,
guaranteeing both are on the exact same clock source. Zero drift.

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

src/engine/audioEngine.js

@@ -35,8 +35,14 @@ export function unsubscribeTick(id) {
 
 function startMasterClock() {
   if (_masterClock) return;
-  _masterTime = Tone.now();
+  _masterTime = 0; // Will be set from first tick
+  let _started = false;
   _masterClock = new Tone.Clock((time) => {
+    // Capture start time from the FIRST callback — guarantees same clock source
+    if (!_started) {
+      _masterTime = time;
+      _started = true;
+    }
     const elapsed = time - _masterTime;
     for (const cb of _tickListeners.values()) {
       cb(time, elapsed);