release: promote develop to main

[britle20]

Summary

• Promote the current develop branch to main for the first integrated mini DAW prototype state.
• Includes repository workflow docs, GitHub templates, Vite + React + TypeScript scaffold, CSS Modules UI, Web Audio engine, model/persistence layers, unit tests, and current feature specs.
• Includes the latest release-prep README and PLANS.md cleanup from docs: prepare develop for main release #62.

Included capability areas

• Browser DAW shell with transport, project sidebar, pattern editor, and song arrangement mode.
• Drum step sequencer, piano roll editing, bundled drum and Iowa Piano samples, and Web Audio playback/scheduling.
• Arrangement clip placement/playback, loop range controls, mixer routing controls, adjustable arrangement length, and WAV export.
• IndexedDB persistence, imported WAV audio clips, and multiple browser-local projects.
• Agentic workflow documentation, code conventions, feature specs, CI, and issue/PR templates.

Testing

• develop branch CI is expected to run on this PR.
• Latest release-prep branch checks passed before merge:
  • npm run typecheck
  • npm run lint
  • npm run test
  • npm run build

Notes / Limitations

• This is still an early browser-based DAW prototype, not a full DAW replacement.
• Project storage is browser-local IndexedDB only.
• No cloud sync, realtime recording, plugin support, or advanced mastering workflow.
• Some project management UI still uses browser prompt/confirm dialogs.

Follow-up

• Continue future feature work from develop after this PR is reviewed and merged.