fix: VCA closes properly with envelope + add CV→Gate module

VCA fix:
- Add cvMod scaler (like oscillator/filter have) so envelope (0-1)
  is scaled by the gain param before modulating VCA
- Zero base gain when CV is connected (in rebuildGraph) so envelope
  = 0 produces silence instead of falling back to base gain
- updateParam keeps cvMod in sync with gain knob

New module: CV→Gate (⚡) in Utility category:
- Converts continuous CV signal (e.g. LFO) to gate on/off
- Threshold knob (0-1, default 0.5): signal above = gate on
- Reads analyser on master clock tick for threshold comparison
- Triggers/releases connected envelopes automatically
- Use case: LFO → CV→Gate → Envelope → VCA for rhythmic gating

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

src/engine/audioEngine.js

@@ -126,13 +126,16 @@ function createNode(mod) {
       };
     }
     case 'vca': {
-      // Use a Multiply node: in × cv
       const gain = new Tone.Gain(p.gain);
+      // CV modulation scaler: envelope (0-1) × gain param → added to gain.gain
+      const cvMod = new Tone.Gain(p.gain);
+      cvMod.connect(gain.gain);
       return {
         node: gain,
-        inputs: { in: gain, cv: gain.gain },
+        _cvMod: cvMod,
+        inputs: { in: gain, cv: cvMod },
         outputs: { out: gain },
-        dispose: () => gain.dispose(),
+        dispose: () => { cvMod.disconnect(); cvMod.dispose(); gain.dispose(); },
       };
     }
     case 'delay': {
@@ -187,6 +190,20 @@ function createNode(mod) {
         dispose: () => analyser.dispose(),
       };
     }
+    case 'cv2gate': {
+      // Converts a continuous CV signal to gate on/off based on threshold.
+      // Uses an analyser to read the CV value and triggers connected envelopes.
+      const analyser = new Tone.Analyser('waveform', 32);
+      const gateSig = new Tone.Signal(0);
+      return {
+        node: analyser,
+        _gateSig: gateSig,
+        _gateState: false,
+        inputs: { in: analyser },
+        outputs: { gate: gateSig },
+        dispose: () => { analyser.dispose(); gateSig.dispose(); },
+      };
+    }
     case 'output': {
       // True stereo output: separate left/right channels → merge → master gain → destination
       const leftGain = new Tone.Gain(1);
@@ -372,7 +389,11 @@ export function updateParam(moduleId, paramName, value) {
       else if (paramName === 'release') entry.node.release = value;
       break;
     case 'vca':
-      if (paramName === 'gain') entry.node.gain.value = value;
+      if (paramName === 'gain') {
+        const hasCV = state.connections.some(c => c.to.moduleId === moduleId && c.to.port === 'cv');
+        if (!hasCV) entry.node.gain.value = value;
+        if (entry._cvMod) entry._cvMod.gain.value = value;
+      }
       break;
     case 'delay':
       if (paramName === 'delayTime') entry.node.delayTime.value = value;
@@ -398,6 +419,7 @@ export function updateParam(moduleId, paramName, value) {
       break;
     case 'keyboard':
     case 'drumpad':
+    case 'cv2gate':
     case 'sequencer':
     case 'pianoroll':
       // All params stored in state, managed by widgets
@@ -517,6 +539,15 @@ export function rebuildGraph() {
     connectWire(conn);
   }
 
+  // Zero base gain on VCAs with active CV connection.
+  // When envelope controls VCA, base gain must be 0 so silence is possible.
+  for (const mod of state.modules) {
+    if (mod.type !== 'vca') continue;
+    const hasCV = state.connections.some(c => c.to.moduleId === mod.id && c.to.port === 'cv');
+    const entry = audioNodes[mod.id];
+    if (entry && hasCV) entry.node.gain.value = 0;
+  }
+
   // Auto-trigger envelopes that have no gate connection (free-running mode).
   // This allows noise/ambient patches to work without a keyboard/sequencer.
   for (const mod of state.modules) {
@@ -531,6 +562,31 @@ export function rebuildGraph() {
       }
     }
   }
+
+  // Register CV→Gate modules on master clock for threshold detection
+  for (const mod of state.modules) {
+    if (mod.type !== 'cv2gate') continue;
+    const entry = audioNodes[mod.id];
+    if (!entry) continue;
+    subscribeTick(`cv2gate-${mod.id}`, () => {
+      const data = entry.node.getValue();
+      const sample = typeof data === 'number' ? data : (data?.[0] ?? 0);
+      const threshold = mod.params?.threshold ?? 0.5;
+      const gateOn = sample > threshold;
+      if (gateOn !== entry._gateState) {
+        entry._gateState = gateOn;
+        entry._gateSig.value = gateOn ? 1 : 0;
+        // Trigger/release connected envelopes
+        for (const conn of getConnectionsFrom(mod.id, 'gate')) {
+          const envEntry = audioNodes[conn.to.moduleId];
+          if (envEntry && envEntry.node instanceof Tone.Envelope) {
+            if (gateOn) envEntry.node.triggerAttack();
+            else envEntry.node.triggerRelease();
+          }
+        }
+      }
+    });
+  }
 }
 
 export function getAnalyserData(moduleId) {

src/engine/moduleRegistry.js

@@ -226,6 +226,23 @@ defineModule('scope', {
   params: {},
 });
 
+// ==================== CV TO GATE ====================
+
+defineModule('cv2gate', {
+  name: 'CV→Gate',
+  icon: '⚡',
+  category: 'Utility',
+  inputs: [
+    { name: 'in', type: PORT_TYPE.CONTROL, label: 'CV In' },
+  ],
+  outputs: [
+    { name: 'gate', type: PORT_TYPE.TRIGGER, label: 'Gate' },
+  ],
+  params: {
+    threshold: { type: 'knob', min: 0, max: 1, default: 0.5, unit: '', label: 'Thresh' },
+  },
+});
+
 // ==================== OUTPUT ====================
 
 defineModule('output', {