Render block-insert pending state

[adamsilverstein]

What

Adds the visual treatment for block-insert-after suggestions (#77434, task 6b). Pairs with the mechanism PR (#77971).

• A block flagged with metadata.suggestion = { type: 'pending-insert' } renders at 70% opacity with a dashed green outline.
• The notes sidebar shows an "Insert block: paragraph" summary line and an underlined preview of the inserted block's content.

How

• style.scss: pending-insert styling (70% opacity + dashed green outline).
• suggestion-summary.js: handles block-insert-after ops with an "Insert block: " line.
• suggestion-diff.js: BlockInsertDiff renders the captured snapshot via collectBlockText with <ins> styling.

Testing

• 137 unit tests pass. Lint and stylelint clean.
• For full-stack testing instructions, see the foundations PR (#77967).

Stack

1. #77967 — Schema v2 bump (foundations)
2. #77968 — Capture block-remove (mechanism)
3. #77970 — Render block-remove preview (UI)
4. #77971 — Capture block-insert-after (mechanism)
5. #77973 — Render block-insert-after preview (UI)
6. #77978 — Capture block-move (mechanism)
7. #77979 — Render block-move preview (UI)

Refs #77434, #73411.

[github-actions]

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[github-actions]

Size Change: +98 B (0%)

Total Size: 7.52 MB

📦 View Changed

Filename	Size	Change
build/scripts/editor/index.min.js	485 kB	+7 B (0%)
build/styles/editor/style-rtl.css	31.1 kB	+24 B (+0.08%)
build/styles/editor/style-rtl.min.css	26.5 kB	+21 B (+0.08%)
build/styles/editor/style.css	31.2 kB	+25 B (+0.08%)
build/styles/editor/style.min.css	26.5 kB	+21 B (+0.08%)

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[github-actions]

Flaky tests detected in 6125134.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/28184704005
📝 Reported issues:

• [Flaky Test] Should save the changes #78689 in /test/e2e/specs/editor/plugins/wp-editor-meta-box.spec.js
• [Flaky Test] shows post-locked modal when document size limit is exceeded #78753 in /test/e2e/specs/editor/collaboration/http-only/collaboration-document-size-lock.spec.ts

[adamsilverstein]

Superseded by the fresh-stack restructure of Suggest mode (#73411).

The feature has been re-sliced from this 16-deep stack onto current trunk as a clean 2-PR stack:

• Suggest mode: core subsystem (intent modes, overlay capture, structural + attribute suggestions) #79713 — Suggest mode: core (intent modes, overlay capture, structural + attribute suggestions, REST/PHP, auto-save, sidebar, summary)
• Suggest mode: inline text and formatting suggestions via markers #79714 — Suggest mode: inline text suggestions (the <mark> marker layer)

The whole feature now sits behind a Suggestion Mode experiment (Settings → Experiments → Collaboration). Closing in favour of the new stack — details: #73411 (comment)