feat: add sampler sustain playback

[britle20]

Summary

Closes #23

Adds a reusable sampler sustain path for sample-based pitched instruments. Iowa Piano now uses serializable sample-zone sustain metadata and the browser audio engine validates that metadata at runtime before enabling native Web Audio forward looping.

Changes

• Added SampleZone.sustain, SampleZone.envelope, and sampleEndSeconds metadata shape for sample-based pitched instruments.
• Added Iowa Piano forward-loop sustain metadata and basic envelope metadata through the shared sample-zone model.
• Added src/audio/sampler-sustain.ts with pure helpers for playback plan resolution, loop validation, envelope clamping, and transport release timing.
• Updated BrowserAudioEngine to schedule sampled notes through the shared sampler plan instead of flat Iowa-specific loop fields.
• Added release cleanup for active/future note voices on pause/stop.
• Added focused unit tests for sampler metadata, loop fallback decisions, envelope clamping, and release timing.
• Updated audio engine, data model, testing 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

Additional checks:

• git diff --check
• Dev server smoke check: Vite served http://127.0.0.1:5174/ with HTTP 200 because port 5173 was occupied.

Screenshots

No UI changes.

Notes / Limitations

• Manual listening checks were not performed in this environment.
• crossfade-loop metadata is recognized as serializable metadata but intentionally falls back to one-shot behavior until a future task implements and tests overlapping crossfade sources.
• Iowa Piano loop metadata is a pragmatic first pass. The engine rejects invalid loop metadata rather than clamping unsafe values into a loop, so any weak samples may still fall back to one-shot playback.
• No user sample import UI, loop point editor, automatic loop detection, arrangement playback, or new runtime dependencies were added.

Follow-up

• Manually test short and long Iowa Piano notes in the browser and tune loop metadata if individual samples sound weak.
• Consider a future loop-point editor or better sample material before crossfade looping.