Add block anatomy plugin

[sneridagh]

Summary

• add a shared block anatomy resolver and a dedicated Plate BlockAnatomyPlugin for the .block.block-.category- contract
• rename registry-backed Plate block nodes from unknown to ploneBlock and update Somersault editor/render paths
• separate anatomy classes/data attributes from StyleFieldsPlugin style-variable resolution
• add Plate-native block categories and update classic @plone/layout BlockWrapper rendering
• fix CMSUI Somersault editor sidebar/editor state around ploneBlock selection and edited-content changes
• document the block anatomy contract and correct block-width/style-field docs
• add towncrier items for blocks, cmsui, helpers, layout, plate, and types

Validation

• pnpm --filter @plone/helpers build
• pnpm --filter @plone/helpers test --run src/blockAnatomy.test.ts src/styleFields.test.ts
• pnpm --filter @plone/layout test --run blocks/BlockWrapper.test.tsx
• pnpm --filter @plone/plate test --run components/editor/plugins/block-anatomy-plugin.test.ts components/editor/plugins/block-width-plugin.test.ts
• pnpm --filter @plone/plate check:ts
• pnpm --filter @plone/layout check:ts
• pnpm --filter @plone/blocks check:ts
• pnpm --filter @plone/cmsui check:ts
• pnpm exec playwright test packages/cmsui/acceptance/tests/block-width.test.ts packages/cmsui/acceptance/tests/image-block.test.ts packages/cmsui/acceptance/tests/teaser-block.test.ts packages/cmsui/acceptance/tests/somersault-renderer.test.ts --config=playwright.config.ts --project=chromium --reporter=line
• git diff --check

---

📚 Documentation preview 📚: https://plone-aurora--102.org.readthedocs.build/

[sneridagh]

@pnicolli I think this was very much needed.
@danalvrz this completes the BMv3 story in Aurora, please check it out!

[sneridagh]

@pnicolli I added some things I had in store from yesterday evening that I wanted to polish. I think now it's ready.

[sneridagh]

@pnicolli if you don't mind, I will go ahead with this one. Do you think it would be intrusive for the current PRs? I took a look and I think nobody has been working on blocks (and their anatomy). So we are good. This would unblock me for taking care of other things.