[codex] Add MDX source highlighting

[lzrscg]

Summary

• Add source-mode MDX syntax highlighting for import/export lines, JSX-style tags and attributes, strings, punctuation, and cspec embed expressions.
• Keep the highlighting active only in source mode so hybrid widgets remain unchanged.
• Add an editor test assertion that source mode renders MDX highlight spans while hiding hybrid widgets.

Validation

• npm run check -w @modularcloud/cspec-editor
• npm run test -w @modularcloud/cspec-editor
• Browser verification of the editor source view at the local Vite app.