Suggest mode 2/7: suggestion storage, REST controller, and provider

[adamsilverstein]

Part of #73411

What's in this PR

Adds the comment-meta storage backbone: _wp_suggestion payload meta and
sanitization (64KB cap, KSES on block snapshots), the 7.1 REST comment
controller subclass scoped to suggestion lifecycle updates, the
comment-meta SuggestionsProvider (create/update/delete, apply/reject for
attribute and structural operations, schema versioning + migration), the
pending-suggestion overlay store with its debounced write queue, and the
auto-save loop that persists overlay entries as note comments. Inline
(marker-based) operations are added by a later layer. Nothing captures
suggestions yet - that starts in the next layer.

Diagram

There is no UI in this layer. It is the write path that turns an edit into a stored suggestion, plus the read path back out:

Testing

This is one layer of the stack. To exercise the whole feature, #78994 bundles all seven layers into one branch and builds it in Playground:

👉 https://playground.wordpress.net/gutenberg.html?pr=78994

Enable Gutenberg > Experiments > Collaboration > Suggestion Mode, then follow the walkthrough in #73411, which also explains how to review the stack layer by layer.

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Suggest mode stack

This rebuilds the manually-stacked Suggest mode work (#73411) as a GitHub Stack of 7 small, independently reviewable PRs, each building on the one below it:

1. #80427 - editor intent (edit/suggest/view) + experiment gate
2. #80428 - suggestion storage, REST controller, provider
3. #80429 - block-level capture (attribute + structural)
4. #80430 - inline marker primitive
5. #80431 - inline suggestion operations
6. #80432 - review UI (Apply/Reject sidebar + summary)
7. #80433 - inline live wiring, docs, e2e

The whole feature can be exercised end-to-end via the combined testing branch #78994 (Playground). Behind the "Suggestion Mode" experiment (Gutenberg > Experiments).

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Size Change: +4.04 kB (+0.05%)

Total Size: 7.82 MB

📦 View Changed

Filename	Size	Change
build/scripts/editor/index.min.js	514 kB	+4.04 kB (+0.79%)

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