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/state.js

@@ -4,6 +4,7 @@
  */
 import { playConnect, playDisconnect, playModuleAdd, playModuleDelete } from './uiSounds.js';
 import { getModuleDef } from './moduleRegistry.js';
+import { invalidateConnectionCache } from './audioEngine.js';
 
 let _listeners = new Set();
 let _nextModuleId = 1;
@@ -93,6 +94,7 @@ export function addConnection(fromModuleId, fromPort, toModuleId, toPort) {
 
   const id = _nextConnectionId++;
   state.connections.push({ id, from: { moduleId: fromModuleId, port: fromPort }, to: { moduleId: toModuleId, port: toPort } });
+  invalidateConnectionCache();
   emit();
   playConnect();
   return id;
@@ -100,6 +102,7 @@ export function addConnection(fromModuleId, fromPort, toModuleId, toPort) {
 
 export function removeConnection(id, _silent = false) {
   state.connections = state.connections.filter(c => c.id !== id);
+  invalidateConnectionCache();
   emit();
   if (!_silent) playDisconnect();
 }