Function lint_markdown_semantics 

pub fn lint_markdown_semantics(content: &str) -> Vec<MarkdownSemanticIssue>

Lint markdown/text for semantic hazards that commonly break context tooling.

This is intentionally lightweight and deterministic:

• rejects unescaped mention-like @ tokens (@foo, @path/to/file)
• flags unterminated fenced code blocks
• flags malformed markdown table rows with inconsistent column counts