docs: plan arrangement mixer panel

[britle20]

Summary

• Adds a feature spec for a bottom dock arrangement mixer panel UI shell.
• Documents the mixer as SONG/arrangement-level UI rather than PAT clip-editor UI.
• Separates the UI shell from future audio routing, metering, and effects work.

Changes

• Added docs/features/14-arrangement-mixer-panel-ui-shell.md.
• Updated PLANS.md Active Issue Order with issue 34 after pending issue 32.
• Updated docs/ui-design.md with bottom dock mixer panel guidance.
• Updated docs/architecture.md and docs/audio-engine.md with future mixer runtime boundaries.
• Updated docs/data-model.md with future serializable mixer state guidance.
• Updated docs/testing.md with mixer UI/routing test considerations.

Testing

• git diff --check
• checked referenced feature doc paths in PLANS.md
• 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 only; no production mixer UI is included.
• The first mixer panel is scoped as a UI shell with local/mock state.
• Real audio routing, real metering, mute/solo audio behavior, and effects are deferred.
• References Arrangement mixer panel UI shell #34.