format: Allow HTML/JSX-like syntax in `@code` blocks

[jack5github]

Affected surface

source syntax

Use case

Documents that demonstrate and/or explain HTML, JSX or XML code, such as AI output regarding changes made to such documents and breakdowns of components of language libraries. It should be possible to use HTML/JSX-like syntax inside @code blocks regardless of the specified language.

Why the current format cannot cover it

• Attempted to use HTML/XML elements in @code lang="html" block. Error: @code contains raw HTML/JSX-like syntax

Required updates

• Spec text updated
• AST schema updated when shape changes
• Language contract updated when language fields change
• Profile contract updated when profile behavior changes
• Valid fixtures added
• Invalid fixtures added when rejection behavior matters
• JavaScript, Python, and Rust behavior updated together
• Conformance updated
• Security model stated for renderer or profile changes

Compatibility impact

Compatible language change. Existing documents will remain valid. Rendered output should be unaffected as content inside @code blocks should be treated as pre-formatted code.