feat: add sequencer, piano roll modules with pre-composed chiptune melody

Add step sequencer (16-step with note/gate editing) and piano roll
(canvas-based MIDI editor with draw/erase tools). Includes a Megaman-style
melody in C minor. Chiptune preset now uses piano roll instead of keyboard.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/engine/audioEngine.js

@@ -146,7 +146,6 @@ function createNode(mod) {
       };
     }
     case 'keyboard': {
-      // Keyboard outputs frequency as a Signal and gate as a Signal
       const freqSig = new Tone.Signal(440);
       const gateSig = new Tone.Signal(0);
       return {
@@ -158,6 +157,37 @@ function createNode(mod) {
         dispose: () => { freqSig.dispose(); gateSig.dispose(); },
       };
     }
+    case 'sequencer': {
+      const freqSig = new Tone.Signal(440);
+      const gateSig = new Tone.Signal(0);
+      // Sequencer loop managed externally by SequencerWidget
+      return {
+        node: null,
+        inputs: {},
+        outputs: { freq: freqSig, gate: gateSig },
+        _freqSig: freqSig,
+        _gateSig: gateSig,
+        _seq: null, // Tone.Sequence set by widget
+        dispose: () => {
+          freqSig.dispose(); gateSig.dispose();
+        },
+      };
+    }
+    case 'pianoroll': {
+      const freqSig = new Tone.Signal(440);
+      const gateSig = new Tone.Signal(0);
+      return {
+        node: null,
+        inputs: {},
+        outputs: { freq: freqSig, gate: gateSig },
+        _freqSig: freqSig,
+        _gateSig: gateSig,
+        _part: null, // Tone.Part set by widget
+        dispose: () => {
+          freqSig.dispose(); gateSig.dispose();
+        },
+      };
+    }
     default:
       return null;
   }
@@ -280,11 +310,31 @@ export function updateParam(moduleId, paramName, value) {
       if (paramName === 'volume') entry.node.gain.value = Tone.dbToGain(value);
       break;
     case 'keyboard':
-      if (paramName === 'octave') { /* stored in state only */ }
+    case 'sequencer':
+    case 'pianoroll':
+      // All params stored in state, managed by widgets
       break;
   }
 }
 
+export function setSequencerSignals(moduleId, freq, gate) {
+  const entry = audioNodes[moduleId];
+  if (!entry) return;
+  if (entry._freqSig) entry._freqSig.value = freq;
+  if (entry._gateSig) entry._gateSig.value = gate ? 1 : 0;
+
+  // Trigger connected envelopes
+  for (const conn of state.connections) {
+    if (conn.from.moduleId === moduleId && conn.from.port === 'gate') {
+      const envEntry = audioNodes[conn.to.moduleId];
+      if (envEntry && envEntry.node instanceof Tone.Envelope) {
+        if (gate) envEntry.node.triggerAttack();
+        else envEntry.node.triggerRelease();
+      }
+    }
+  }
+}
+
 export function triggerKeyboard(moduleId, freq, gate) {
   const entry = audioNodes[moduleId];
   if (!entry) return;