feat: implement basic piano roll

[britle20]

Summary

Implements the basic 1-bar piano roll editor for the selected hybrid clip.

Closes #2.

Changes

• Added C4-C5 piano roll pitch rows using the bundled Iowa Piano sample range.
• Added left-click/drag note creation, note drag movement, and right-click deletion.
• Stores note events in the selected clip as serializable midiNote, startTick, durationTicks, and velocity.
• Added synth note loop playback through the audio engine so held notes can play for their tick duration.
• Added bundled Iowa Piano sample metadata and committed the 13 C4-C5 WAV files under public/samples/pitched_instruments/Iowa_Piano/.
• Synchronized piano keyboard vertical scrolling with the note grid and kept the beat ruler sticky.
• Renamed visible instrument labels from Lead Synth to Iowa Piano.
• Added unit tests for piano roll model transformations and bundled piano sample metadata.
• Updated data model, audio engine, UI design, and feature spec docs.

Testing

• git diff --check
• npm run typecheck --if-present
• npm run lint --if-present
• npm run test --if-present
• npm run build --if-present

Manual Check

• Started Vite dev server. Port 5173 was occupied, so Vite served on http://127.0.0.1:5174/.
• Confirmed the dev server returned HTTP 200 for the app root.

Notes / Limitations

• Piano roll playback uses a basic oscillator synth for held-note duration.
• The bundled Iowa Piano WAV files are registered as sample metadata and define the initial C4-C5 range, but a full sampler instrument is not implemented yet.
• Dedicated note resize handles are not implemented; left-drag on empty grid creates longer notes, and existing notes can be moved by dragging.
• Pause/resume and visual playhead behavior remain scoped to Transport pause/resume and editor playhead #16.