docs: plan arrangement clip placement playback

[britle20]

Summary

Adds planning documentation for arrangement clip placement and SONG mode playback.

Refs #41.

Changes

• Added docs/features/16-arrangement-clip-placement-and-playback.md.
• Updated PLANS.md Active Issue Order to place Arrangement clip placement and playback #41 after WAV file import as audio clip #39.
• Updated README feature map and roadmap.
• Updated product, architecture, data model, audio engine, UI, and testing docs for arrangement placement/playback responsibilities.

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 is documentation and issue planning only. No production arrangement code is implemented here.
• Playback is included in the implementation scope for Arrangement clip placement and playback #41.
• Imported audio clip placement/playback depends on WAV file import as audio clip #39. If WAV file import as audio clip #39 is not implemented first, Arrangement clip placement and playback #41 should guard the audio-clip path and still support hybrid clip placement/playback.
• Time stretching, clip resizing, arrangement loop regions, imported file persistence, and mixer routing are deferred.