Suggest mode: Suggestion summary, attribute conflict detection, and architecture docs

[adamsilverstein]

Overview

One of three replacement PRs for the now-closed #77407. This slice introduces the suggestion summary renderer, per-attribute conflict detection, snackbar feedback, and architecture documentation.

Tracking issue: #73411.

What's in this PR

Suggestion-mode component additions

• suggestion-summary.js — Compact alternative to the full inline diff. Renders an Add / Delete / Format line per operation suitable for sidebar threads. Used by the collab-sidebar PR on top of this branch.
• hasAttributeConflict + parseSuggestionPayload (provider) — Replaces the post-modified_gmt staleness compare with a per-attribute divergence check. modified_gmt flagged nearly every auto-saved suggestion as stale because every save bumps the post timestamp; the new check only prompts when the specific attributes a suggestion targets have moved off baseline.
• with-suggestion-overlay and suggestion-diff refinements to match the new contract.

Editor store / UX

• setEditorIntent snackbar — Mode transitions now surface a snackbar ("You're suggesting" / "You're editing" / "You're viewing") alongside the existing a11y announcement.
• Store wiring — actions.js, reducer.js, selectors.js updated for the new selectors; use-block-editor-settings.js wires the suggestion provider settings through.
• Style — suggestion-mode/style.scss is the home for suggestion visuals (block bracket marker, summary card); imported from packages/editor/src/style.scss.

Documentation

• docs/explanations/architecture/suggestions.md — Architecture doc covering the provider abstraction, overlay subsystem, and lifecycle.
• docs/reference-guides/data/data-core-editor.md — Updated to cover the new editor-store selectors / actions.

Tests

• Unit: test/provider.js (extended), test/suggestion-summary.js (new), test/with-suggestion-overlay.js (extended).
• e2e: suggestion-mode.spec.js updated to exercise the new flow.

Test plan

npm run test:unit -- packages/editor/src/components/suggestion-mode
npm run test:unit -- packages/editor/src/store/test/actions.js
npm run test:e2e -- test/e2e/specs/editor/various/suggestion-mode.spec.js
npm run lint:js -- packages/editor/src/components/suggestion-mode packages/editor/src/store

Manual:

1. Switch to Suggest mode → snackbar reads "You're suggesting".
2. Edit a paragraph → SuggestionSummary renders in the inline diff and (after the sidebar PR lands) in the note thread.
3. Submit, then in another window edit the same paragraph's attributes → reopening the original suggestion in the sidebar shows the staleness confirmation only when the specific attributes diverge — not on any unrelated post change.

Replaces part of #77407.

---

🗺️ 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 ← this PR	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	Background debounced save (replaces commit-bar)

📋 Tracking issue: #73411

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