docs: plan main DAW UI shell

[britle20]

Summary

References #11.

Plans the Main DAW UI Shell feature so the Tailwind-based prototype can be converted into the project's CSS Modules architecture in a later implementation PR.

Changes

• Added docs/features/06-main-daw-ui-shell.md.
• Updated PLANS.md Active Issue Order to put Main DAW UI Shell #11 before Lookahead Scheduler #5 Lookahead Scheduler.
• Marked AudioContext and Sample Playback feature spec complete after PR feat: add AudioContext sample playback #10 merged.
• Added UI Reference Policy to AGENTS.md and docs/ui-design.md.
• Added the new feature spec to the README docs map.

Testing

• git diff --check
• Markdown internal reference check
• npm run typecheck
• npm run lint
• npm run test
• npm run build

Notes / Limitations

• Planning-only PR; no production UI implementation is included.
• The future implementation should preserve primitive/semantic design token policy and must not add Tailwind.