feat(webkit): [ENG-47606] add DocProse, the documentation typography contract

[gabriel-lisboa-azion]

Summary

• Adds DocProse under the new documentation category — the typography contract a documentation page wraps its body in. It styles descendants rather than itself, so it works over any markdown/MDX pipeline's output, hand-written markup, or CMS HTML. The design system ships no renderer; the consumer brings their own.
• Every prose rule carries :not([data-doc-chrome],[data-doc-chrome]_*), so the contract stops at a documentation component's edge instead of restyling the webkit internals it wraps (a Message body, a CodeBlock's lines, an Accordion trigger's heading).
• It has no measure of its own — it is pure typography, and the consumer caps the reading column with .layout-column-docs.

How to test

1. pnpm storybook:dev → Components / Documentation / DocProse. The DocProse story shows the full ladder; Chrome boundary shows a data-doc-chrome subtree keeping its own styling while the prose around it does not.
2. In the browser, in both themes, confirm the in-content ladder: h2 32 (48 at xl) / h3 24 / h4 16 / prose 16 / block 24, and that it reads the same at 390px and 1440px. The ladder is pinned by descending responsive pairs, so a rung that moves with width is a bug.
3. cd packages/webkit && npx vitest run src/components/documentation/doc-prose/doc-prose.test.ts — 8/8 pass in Chromium.
4. node packages/webkit/scripts/check-size.mjs — doc-prose 1.02 KB against a 1.5 KB budget.
5. node packages/webkit/scripts/check-authoring.mjs and node .claude/hooks/validate-story-source.mjs --all — both clean.

Notes

• Stacked: base is feat(theme): [ENG-47561] add the docs measure and the prose + mono body registers #905 (theme tokens), which is itself based on chore: [ENG-47606] add the documentation component category #904 (the documentation category). Merge chore: [ENG-47606] add the documentation component category #904 → feat(theme): [ENG-47561] add the docs measure and the prose + mono body registers #905 → this; each retargets automatically.
• p and li take the new text-body-prose-md register rather than text-body-md plus a leading-* override. check-authoring's leading-raw check deliberately rejects that override even in token form, and line-height belongs in the token — the same call text-body-code-sm already makes.
• The test asserts no computed styles, classes or pixels: Vitest browser mode loads no Tailwind, so those pass identically in the broken and fixed states. The ladder is verified by the visual gate and in the browser; the suite covers slot pass-through, attribute forwarding, the testid override and axe. The header comment says so.
• Storybook forces h1..h6 to weight 400 via an unlayered !important in preview.css, so h4's rung — which it earns on font-medium, not on space — is not verifiable in Storybook. Check that one in a real page.
• Related: ENG-47606.

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Delivery

The Documentation family is delivered on the feature branch feat/docs-components-bundle, not piecemeal to main — so main takes the whole family as one merge, while review stays per-component.

Merge order, bottom-up — each PR's base is the one before it, and each retargets automatically as its parent lands:

#904 → #905 → #906 → #907 → #908 → #909 → #910 → #911 → #912 → #913 → #914 → #915 → #916 → #917 → #918 → feat/docs-components-bundle → main

#904	the documentation category (standards only)
#905	theme: the docs measure + the prose and mono body registers
#906–#918	DocProse · DocCallout · DocCard · DocCardGroup · DocItem · DocSteps + DocStep · DocFrame · DocUpdate · DocPrompt · DocTooltip · DocPagination · DocPageHeader · DocOnThisPage