Suggest mode: Auto-save subsystem (Phase 6)

[adamsilverstein]

Overview

Follow-up to #77407 (Phase 5). Carves the auto-save subsystem out of Phase 5 so each PR concentrates on one concern — Phase 5 stays focused on REST permissions + PHP tests, and this PR owns the Google Docs–style background save.

Tracked in #73411.

What's in this PR

• SuggestionAutoSave — invisible component that debounces overlay edits (1.5 s per block) and persists them as note comments with the _wp_suggestion meta payload. Replaces the explicit SuggestionCommitBar toolbar button (restored on Phase 5 so the prior stack stays functional in isolation).
• Per-block save queue — saves on the same clientId chain sequentially so a slow network call can't race with a follow-up edit and produce duplicate POSTs or out-of-order writes. Different blocks run concurrently.
• updateSuggestion / deleteSuggestion on the provider — once a comment id is linked, subsequent edits PUT the same comment; a fully reverted overlay trashes the note. Includes a stale-link check via core-data so a peer-resolved comment doesn't get clobbered mid-session.
• Overlay plumbing — commentId and syncedOpsKey fields on overlay entries, SET_COMMENT_ID / SET_SYNCED_OPS_KEY reducer cases, and matching setCommentId / setSyncedOpsKey callbacks on the context.
• Tests — `test/auto-save.js` covers debounce, queue ordering, create/update/delete branching, and stale-link orphan handling. `test/overlay-context.js` covers the new reducer cases.

Test plan

1. Enter Suggest mode and edit a paragraph.
2. After ~1.5 s the network shows a `POST /wp/v2/comments` creating the note.
3. Continue typing — subsequent saves are `PUT`s against the same comment id.
4. Revert the overlay to baseline — the note is trashed.
5. Have a peer accept the suggestion mid-session — the next local edit creates a fresh comment instead of clobbering the resolved one.

```bash
npm run test:unit -- packages/editor/src/components/suggestion-mode/
```

Stack

🗺️ PR Stack Navigation

#	PR	Phase
1	#77403 Intent scaffolding	Edit / Suggest / View mode
2	#77404 Overlay capture	In-memory suggestion overlay
3	#77405 Provider + Accept/Reject	_wp_suggestion meta, provider, sidebar actions
4	#77406 Summary + docs + attribute tests	Add/Delete/Formatting summary, architecture stub, conflict scoping
5a	#78351 REST permissions and PHP coverage	Permissions, payload cap, PHP tests
5b	#78352 Summary + attribute conflict + docs	Renderer, per-attribute staleness, architecture docs
5c	#78353 Surface Apply/Reject in the collaboration sidebar	Icon buttons + e2e + sidebar wiring
6	#78308 Auto-save subsystem ← this PR	Background debounced save (replaces commit-bar)

📋 Tracking issue: #73411

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Co-authored-by: adamsilverstein <adamsilverstein@git.wordpress.org>

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