docs: plan mixer audio routing controls

[britle20]

Summary

Adds planning documentation for turning the mixer panel from a UI shell into real audio routing and track controls.

Refs #43.

Changes

• Added docs/features/17-mixer-audio-routing-and-track-controls.md.
• Added Mixer audio routing and track controls #43 to PLANS.md Active Issue Order after arrangement placement/playback.
• Updated README feature map and roadmap.
• Updated architecture, data model, audio engine, UI, product, and testing docs for functional mixer routing.
• Marked the mixer panel UI shell feature as Done.

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 and feature planning only. No production mixer code is implemented here.
• The first functional mixer scope is track/master volume, mute, solo, real routing, and runtime meters.
• Audio effects, pan, sends, buses, automation, and export rendering are deferred.
• Mixer audio routing and track controls #43 depends on Arrangement clip placement and playback #41 because real mixer routing needs arrangement playback sources to carry track identity.