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/components/ModuleNode.jsx

@@ -5,6 +5,8 @@ import { updateParam } from '../engine/audioEngine.js';
 import Knob from './Knob.jsx';
 import ScopeDisplay from './ScopeDisplay.jsx';
 import KeyboardWidget from './KeyboardWidget.jsx';
+import SequencerWidget from './SequencerWidget.jsx';
+import PianoRollWidget from './PianoRollWidget.jsx';
 
 export default function ModuleNode({ mod, zoom, onStartConnect, onPortPosition }) {
   const def = getModuleDef(mod.type);
@@ -65,7 +67,11 @@ export default function ModuleNode({ mod, zoom, onStartConnect, onPortPosition }
   return (
     <div
       className={`module ${isSelected ? 'selected' : ''}`}
-      style={{ left: mod.x * zoom, top: mod.y * zoom, transform: `scale(${zoom})`, transformOrigin: 'top left' }}
+      style={{
+        left: mod.x * zoom, top: mod.y * zoom,
+        transform: `scale(${zoom})`, transformOrigin: 'top left',
+        ...(mod.type === 'pianoroll' ? { width: 520 } : mod.type === 'sequencer' ? { width: 310 } : {}),
+      }}
       data-module-id={mod.id}
       onPointerDown={(e) => {
         // Don't deselect when clicking inside a module
@@ -142,6 +148,12 @@ export default function ModuleNode({ mod, zoom, onStartConnect, onPortPosition }
         {/* Keyboard widget */}
         {mod.type === 'keyboard' && <KeyboardWidget moduleId={mod.id} />}
 
+        {/* Sequencer widget */}
+        {mod.type === 'sequencer' && <SequencerWidget moduleId={mod.id} />}
+
+        {/* Piano Roll widget */}
+        {mod.type === 'pianoroll' && <PianoRollWidget moduleId={mod.id} />}
+
         {/* Output ports */}
         {def.outputs.map(port => (
           <div key={port.name} className="port-row output">