Add the block content classname contract foundations (#200 Phase 1a)

[sneridagh]

Phase 1a of #200: the foundations of the block content classname contract, with zero visual change.

Stacked on #205 (#199 A3), which is on #204 and #203. The base is a3-content-css-pilot. This PR's own change is only the last commit.

Why Phase 1 is split

#200's Phase 1 mixes zero-diff groundwork with changes that visibly alter spacing:

• Anatomy gaps: adding hr to plateBlocksConfig gives it .category-separator, which @plone/layout's content-area.css already styles.
• Shared spacing rules: moving them from content-area.css into content.css settles the view/edit drift.

So the phase is split:

• 1a (this PR): the guardrails, the list parts and the docs. Zero diff.
• 1b (next): the hr / h1 anatomy and the shared spacing rules, with every accepted diff listed.

Changes

Class-contract test (plate/acceptance/tests/content-class-contract.test.ts)

• What it checks: it renders every native-block fixture in the public view and collects every class inside the content, grouped by the Plate node that renders it (p, h2, table, code, …, and editor for the editor root).
• What's allowed:
  • the contract: slate-*, block, block-<type>, block-<type>__<part>, category-*;
  • third-party classes rendered inside the content: highlight.js (hljs-*, function_), lucide icons, React Aria (react-aria-*).
• PENDING map: every remaining class must match the map of Tailwind classes still waiting for conversion, per node, exactly.
  • Ratchet: a new utility anywhere fails the test, and so does a converted node whose entry wasn't removed. The map can only shrink; each later phase deletes its entries.
• Where it runs: it's an acceptance test rather than a unit test. The somersault renderer depends on the app's full registry config, and acceptance tests run on every PR in CI, so it still gates each phase.
• Second test: the list hooks below render as expected.

Stylelint rules for styles/content.css

These are overrides in the .stylelintrc of @plone/plate, @plone/blocks and @plone/agave, the packages that ship one. They enforce #199's authoring rules:

• at-rule-disallowed-list: layer;
• selector-disallowed-list (with splitList) for :root, html, body and selectors that start with a bare element, including inside :where() / :is().

I checked them against a sample: it flags @layer, :root, html, body .x, h1, figure img and :where(figure img), and it allows .ok h1, :where(.x) and .content-area. All three real content.css files pass.

List parts

• Part classes: block-p__list on ul / ol and block-p__item on li, in both the static and the editable list renderers.
• Data attribute: BlockAnatomyPlugin adds data-list-style-type to list blocks (paragraphs with a listStyleType), with a unit test.
• No CSS targets them yet, so nothing changes visually. Phase 4 (lists) uses them.

Docs

• docs/development/block-anatomy.md gets a "Classname contract for themers" section: hooks, rules, shared type names (.block-video.slate-video / .slate-ploneBlock), BEM parts, variants as data attributes, and lists.
• @plone/plate AGENTS.md: how to maintain PENDING.
• @plone/blocks AGENTS.md: "keep CSS colocated" now points block content styles to styles/content.css.

Validation

• CI=1 pnpm visual-test --retries=0: 22 passed, pixel-identical to the reference.
• pnpm acceptance-test: 168 passed, including the 2 new contract tests.
• pnpm --filter @plone/plate test --run: 75 passed, 1 of them new. check:ts, eslint and stylelint: clean.

Part of #200.