feat: add pitched instrument selection

[britle20]

Summary

Closes #19

Adds clip-level pitched instrument lanes for Default Synth and Iowa Piano, with note events owned by serializable instrumentId values so both instruments can coexist and play together in one clip.

Changes

• Replaces the piano-roll-local instrument toggle with sidebar clip child entries for Default Synth and Iowa Piano.
• Adds instrumentId to serializable NoteEvent data and filters the visible piano roll notes by the selected sidebar instrument.
• Schedules all note events in the selected clip, allowing Default Synth and Iowa Piano to sound simultaneously.
• Defines serializable pitched instrument metadata and Iowa Piano C4-C5 sample zones.
• Adds Iowa Piano sample playback with gain envelopes and a PCM WAV decode fallback when decodeAudioData cannot decode a bundled WAV.
• Adds per-sample sampleStartSeconds offsets so Iowa Piano playback skips leading silence in the bundled one-second WAV files.
• Disables Iowa Piano sustain looping for this PR to avoid long notes sounding like repeated strikes.
• Updates docs and tests for instrument-owned notes, sample start offsets, and deferred advanced sampler sustain.

Testing

• npm run typecheck --if-present
• npm run lint --if-present
• npm run test --if-present
• npm run build --if-present

Screenshots

Not included. Dev server was previously started and the app returned HTTP 200 at http://127.0.0.1:5174/ because port 5173 was already in use.

Notes / Limitations

• The bundled Iowa Piano samples contain substantial leading silence before the audible attack. Short notes could be stopped before the attack, so this PR starts sample playback at explicit offsets.
• Iowa Piano samples currently play once and do not use AudioBufferSourceNode.loop.
• Long Iowa Piano notes may naturally decay or end instead of sustaining for the full note duration.
• Advanced sampler sustain, loop point tuning, and crossfaded looping should be handled in a follow-up issue after this PR merges.
• Manual audible confirmation still needs browser review.

Follow-up

• After this PR merges, create a follow-up issue for sampler-based advanced sustain.
• Then continue with the next Active Issue Order entry.