fix: reduce master clock to 240 Hz + eliminate note-off timeouts

- Master clock 960→240 Hz: reduces CPU/GC pressure by 4x while still
  providing 12x headroom for 300 BPM sixteenths
- Remove Tone.getContext().setTimeout() for note-off scheduling —
  these accumulated over time causing periodic hiccups
- Note-off now happens at step boundary: previous gate turned off
  at the start of each new step (cleaner, zero accumulation)

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

src/engine/audioEngine.js

@@ -15,7 +15,7 @@ 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
+const MASTER_TICK_RATE = 240; // Hz — enough for 300 BPM sixteenths (20 Hz) with 12x headroom
 let _masterClock = null;
 let _masterTime = 0; // audio-context seconds at clock start
 const _tickListeners = new Map(); // id → callback(audioTime, elapsed)