docs: plan pitched instruments and sustain

[britle20]

Summary

Plans the next pitched-instrument feature: keep the current oscillator playback as Default Synth, add an Iowa Piano sample-based instrument, and attempt long-note sustain with explicit loop metadata and release envelopes.

References #19.

Changes

• Added docs/features/08-pitched-instruments-and-sustain.md.
• Updated PLANS.md so Pitched instrument selection and Iowa Piano sustain #19 follows Transport pause/resume and editor playhead #16 in Active Issue Order.
• Marked Basic Piano Roll as done after PR feat: implement basic piano roll #18 was merged.
• Documented Default Synth, Iowa Piano, sample zones, sustain loop metadata, and runtime-only audio boundaries.
• Updated UI and testing docs for pitched instrument selection and sustain verification.

Testing

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

Notes / Limitations

• This PR is documentation/planning only.
• No production audio engine, sampler, or UI implementation is included.
• Sustain loop point tuning is intentionally scoped to the future implementation PR.