docs: plan persistence length and export features

[britle20]

Summary

Adds planning docs for four separate upcoming feature tasks:

• IndexedDB project persistence (IndexedDB project persistence #47)
• Variable hybrid clip length (Variable hybrid clip length #48)
• Adjustable arrangement length (Adjustable arrangement length #49)
• Arrangement WAV export (Arrangement WAV export #50)

Changes

• Added feature specs under docs/features/18-21.
• Updated PLANS.md Active Issue Order to remove merged Arrangement clip placement and playback #41 and add IndexedDB project persistence #47-Arrangement WAV export #50 after Mixer audio routing and track controls #43.
• Updated README feature links and roadmap.
• Updated architecture, data model, audio engine, UI, and testing docs for persistence, clip length, arrangement length, and WAV export.

Testing

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

Notes / Limitations

• This is planning/documentation only; no production feature code is implemented.
• WAV is documented as the first export format. MP3/compressed export remains out of scope.
• The implementation issues are intentionally separate because they touch different layers and risk areas.

Follow-up

• Continue implementation through Active Issue Order in PLANS.md.
• Keep Arrangement WAV export #50 after the required arrangement/audio routing work so export has a clear source graph to render.