M23-B: Integrate SyntaxHighlighter into MdRenderer

[bug-ops]

Parent: #344
Depends on: #345

Scope

• Add code_lang: Option<String> field to MdRenderer
• Store language tag on Event::Start(Tag::CodeBlock(Fenced(lang)))
• Clear on Event::End(TagEnd::CodeBlock)
• In Event::Text when in_code_block && code_lang.is_some(): call SyntaxHighlighter::highlight()
• If Some(spans) returned, push multiple spans instead of single flat span
• If None, fall back to theme.code_block (current behavior)
• Preserve " " prefix indentation for code lines

Acceptance criteria

• Fenced code blocks with known languages render with multiple colored spans
• Unknown languages fall back to flat code_block style
• Unfenced code blocks unchanged
• Existing tests pass; new test for highlighted code block