[MAIC Editor] Enablement infrastructure (pre-slide-surface)

[wyuc]

Parent: #560. Lands before #562 (slide surface).

Scope

1. Feature flag

• NEXT_PUBLIC_MAIC_EDITOR_ENABLED, default OFF.
• Gates only the Pro toggle in Header; StageMode union unchanged.
• Retained as kill-switch post-rollout.

2. Slide DSL schema versioning

• slide.schemaVersion: number on slide content; current shape is v1.
• migrateSlide(slide) keyed by version; v1 body empty, shape + tests in.
• Slide loader funnels through migrateSlide.
• Tests: pure, idempotent, v1 round-trip is identity.

3. Round-trip integration test harness

• New tests/edit/round-trip/ directory.
• Harness: fixture slide + ops list → apply → PPTX export → reimport → diff.
• Initial cases: noop + one trivial op (e.g. change a text element's content).
• Each [MAIC Editor] Slide surface — first concrete static-display surface #562 element adds its own case.
• Runs as part of pnpm test.

4. Edit history persistence

• Op history (capped at 50 in feat(maic-editor): framework primitives + edit StageMode #564) autosaves to localStorage, keyed per scene.
• On Stage mount: check the key; if present, prompt restore / discard.
• i18n: 2 strings (prompt title + body) × 6 locales.

5. Concurrency guards

• AI regen lock — reverse of feat(maic-editor): framework primitives + edit StageMode #564's auto-exit. Edit mode holding a scene → any regenerate-this-scene call (manual retry, auto-on-failure) is refused (pessimistic; no queue).
• Multi-tab detect — BroadcastChannel with localStorage event fallback. Second tab attempting edit on the same course is refused with a clear message.
• i18n: 2 strings × 6 locales.

Out of scope

• Server-side history sync (Phase 2).
• Cross-device conflict resolution.
• Optimistic regen guard (revisit if pessimistic friction shows up).

Acceptance criteria

• Flag OFF: zero behavior change vs feat/maic-editor-v0 tip
• Flag ON: Pro toggle behaves as in refactor(maic-editor): drop EditModeSidebar; clean Pro mode chrome #568
• slide.schemaVersion in type + serialized; migrateSlide tested
• Round-trip harness with noop + trivial op cases, green in CI
• Refresh mid-edit → per-scene restore prompt on remount; accept restores, dismiss clears
• Regenerate-this-scene refused while edit mode holds it
• Second tab edit on same course refused
• All 6 CI gates green; no new lint warnings beyond pre-existing 7

Note

No new user-visible edit affordances ship here — Pro mode behavior is identical to #568 with flag ON. Tests dominate the diff.