fix: reduce main thread pressure to prevent audio buffer underruns

The periodic audio glitches were caused by main thread starvation:
~840 events/sec during playback starved the audio buffer.

Changes:
- Master clock 480→120 Hz (still 6x headroom for 300 BPM sixteenths)
- Connection cache: replace O(n) reduce hash with dirty flag (zero work
  on cache hit, flag set only when connections actually change)
- Tone.js lookAhead: 100ms→50ms for tighter scheduling
- ModuleNode LFO visualization RAF: 60fps→15fps (every 4th frame)
- ScopeDisplay RAF: 60fps→30fps (every 2nd frame)

Net effect: ~840 events/sec → ~200 events/sec during playback.
Audio processing gets 4x more main thread headroom.

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

src/engine/audioEngine.js

@@ -16,7 +16,7 @@ const keyboardState = { frequency: 440, gate: false };
 // Single clock with integer tick counter. All sequencers/piano rolls
 // derive their step positions from this shared tick count.
 // Using integers avoids floating-point drift entirely.
-export const MASTER_TICK_RATE = 480; // Hz — must be high enough for fastest BPM
+export const MASTER_TICK_RATE = 120; // Hz — 6x headroom for 300 BPM sixteenths (20 Hz). Lower = less main thread pressure.
 let _masterClock = null;
 const _tickListeners = new Map(); // id → callback(audioTime, ticks)
 
@@ -406,21 +406,23 @@ export function updateParam(moduleId, paramName, value) {
 }
 
 // Cache connection lookups for hot-path audio scheduling
-// Rebuilt lazily when connections change
-let _connCacheVersion = -1;
+// Rebuilt only when connections actually change (dirty flag, no computation on hit)
+let _connCacheDirty = true;
 const _connByModulePort = new Map(); // "moduleId-portName" → [connections]
 
+export function invalidateConnectionCache() {
+  _connCacheDirty = true;
+}
+
 function getConnectionsFrom(moduleId, portName) {
-  // Rebuild cache if connections changed
-  const version = state.connections.length + state.connections.reduce((s, c) => s + c.id, 0);
-  if (version !== _connCacheVersion) {
+  if (_connCacheDirty) {
     _connByModulePort.clear();
     for (const conn of state.connections) {
       const key = `${conn.from.moduleId}-${conn.from.port}`;
       if (!_connByModulePort.has(key)) _connByModulePort.set(key, []);
       _connByModulePort.get(key).push(conn);
     }
-    _connCacheVersion = version;
+    _connCacheDirty = false;
   }
   return _connByModulePort.get(`${moduleId}-${portName}`) || [];
 }