feat: add sidebar clip instrument management

[britle20]

Summary

• Implements editable sidebar clip management for create, select, rename, and delete.
• Adds per-clip pitched instrument membership with add/remove controls for Default Synth and Iowa Piano.
• Keeps Drums mandatory and keeps at least one pitched instrument per clip so piano roll state remains visible and serializable.
• Updates model helpers, unit tests, data-model docs, feature status, and Active Issue Order.

Testing

• npm run typecheck --if-present
• npm run lint --if-present
• npm run test --if-present
• npm run build --if-present
• npm run dev -- --host 127.0.0.1 --port 5173, served on 5174 because 5173 was in use, then curl -I http://127.0.0.1:5174/ returned HTTP 200

Notes / Limitations

• Uses browser confirmation dialogs for deleting clips with events and removing instruments with owned notes.
• Does not add persistence, arrangement placement, clip duplication, custom instruments, drum lane add/delete, or import/export.
• Stops before the next issue because Drum step subdivisions #32 is likely to touch nearby clip/drum model and sequencer state while this PR is pending.

Closes #30