Rewrite workshop 08: Extensions #335

Part of #326.

Questions

• Packaging mechanism: enablement, registry flag, separate sync and transport (today flush() syncs fixtures and the preview socket carries light traffic)
• Lights, flows (today "light sequences"), Blackmagic camera control, live picture
• What the removal of Day planning takes with it: daylight windows, offline days, StepThrough, forecast
• Extra fields: removed for now, parked as a future extension mechanism

As-is sources

apps/mobile/src/fixtures/, apps/mobile/src/ble/, DevicesApp.tsx, SequencesApp.tsx, days.ts, fields.ts, docs/features/lights.md, cameras.md, schedule.md, extra-fields.md

Format

No product code. I prepare docs/rewrite/08-extensions.md with the as-is state (with file paths), the problems and leaks, a proposal and the open questions; mockups go to docs/rewrite/mockups/. The owner decides, the decisions are recorded in the page's "Decided" section and here, and the PR is merged into rewrite/plan.