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/components/SequencerWidget.jsx

@@ -71,18 +71,23 @@ export default function SequencerWidget({ moduleId }) {
     }
 
     let lastStepIdx = -1;
-    let noteOffTimeout = null;
+    let lastGateOn = false;
 
     subscribeTick(`seq-${moduleId}`, (time, elapsed) => {
       const currentBpm = bpmRef.current;
       const currentNumSteps = numStepsRef.current;
       const sixteenthRate = (currentBpm * 4) / 60;
-      const stepDuration = 1 / sixteenthRate;
       const stepIdx = Math.floor(elapsed * sixteenthRate) % currentNumSteps;
 
       if (stepIdx === lastStepIdx) return;
       lastStepIdx = stepIdx;
 
+      // Turn off previous note at step boundary (no setTimeout needed)
+      if (lastGateOn) {
+        setSequencerSignals(moduleId, 0, false);
+        lastGateOn = false;
+      }
+
       const s = stepsRef.current[stepIdx];
       if (!s) return;
 
@@ -90,11 +95,7 @@ export default function SequencerWidget({ moduleId }) {
 
       if (s.gate) {
         setSequencerSignals(moduleId, midiToFreq(s.midi), true);
-        Tone.getContext().setTimeout(() => {
+        lastGateOn = true;
-          setSequencerSignals(moduleId, midiToFreq(s.midi), false);
-        }, stepDuration * 0.8);
-      } else {
-        setSequencerSignals(moduleId, midiToFreq(s.midi), false);
       }
     });
 

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)