Add the styles/content.css entry point to the add-on styles loader (#199 A2)

[sneridagh]

Step A2 of #199: a styles/content.css entry point in the add-on styles loader, so add-ons write block styles once and get them in both the Public UI and the CMSUI.

Stacked on #203 (A1). The base is a1-css-layer-order. Retarget to main once #203 is merged. This PR's own change is only the last commit.

Changes

• @plone/registry (create-addons-styles-loader.ts):

  • New generated file: .plone/content.css aggregates every add-on's styles/content.css, in add-on order. It emits no Tailwind @source lines, because content CSS is plain CSS by contract.

  • Imported by both UIs: the generated .plone/publicui.css and .plone/cmsui.css now start with:

    @import './content.css' layer(plone-content);

  • Exports: buildContentLoaderCode, plus CONTENT_STYLESHEET and CONTENT_LAYER constants. styles/publicui.css and styles/cmsui.css work exactly as before.

• Tests: a new __tests__/create-addons-styles-loader.test.js. Until now the styles loader had no tests. It covers:
  • no add-on shipping content styles;
  • several add-ons, in add-on order;
  • no @source lines in content.css, even for add-ons that use Tailwind;
  • both UI loaders importing content.css first, in plone-content;
  • @source still emitted for Tailwind add-ons in the UI loaders;
  • all three files written to .plone/.
• Docs (docs/conceptual-guides/add-on-styles-loader.md): a "Content styles" section covering:
  • the convention and the layer order;
  • the base-layer convention for theme resets;
  • the authoring rules;
  • "Override block styles from an add-on", with examples and a warning about the @layer-inside-content.css trap.

No add-on ships a styles/content.css yet. A3 adds the first ones, so this PR changes no rendered CSS: the content import is an empty file.

Verified end to end

I built the app with temporary marker rules in @plone/plate/styles/content.css (:where(.a2-marker)) and @plone/agave/styles/content.css (.a2-marker). Both built bundles then contain:

@layer plone-content{:where(.a2-marker){color:#010203}.a2-marker{color:#040506}}

So both UIs get the content styles, in the plone-content layer, with the plate rule before the Agave rule. Tailwind v4 and Vite handle @import … layer() in the generated files, which settles the risk noted in #199. The markers aren't committed.

Validation

• pnpm --filter @plone/registry test --run: 125 passed, 7 of them new.
• pnpm --filter @plone/registry check:ts and build: clean.
• pnpm --filter @plone/aurora build: clean, with and without add-on content styles.
• pnpm --filter @plone/registry check:exports fails, but with the same node10 resolution and CJS/ESM warnings on every subpath export of the package. That looks pre-existing and unrelated to this change.
• Vale couldn't run locally (its styles aren't installed in this checkout), so the docs check is left to CI.

Part of #199.