Render task-list checkboxes in formal mode

[p13i]

Gap

Task-list markers such as - [ ] and - [x] currently
render as literal item text. They are not part of the
documented formal subset.

Acceptance criteria

• Add neutral checked and unchecked fixtures.
• Render non-interactive, accessible checkbox states without
  permitting raw HTML.
• Preserve ordinary list rendering.
• Document the added subset behavior.

Verification

make check

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