Function try_regex_fallback 

pub fn try_regex_fallback(
    source: &str,
    language: &str,
) -> Option<SemanticAnalysis>

Attempt regex-based extraction for formats without a tree-sitter grammar.

Returns Some(SemanticAnalysis) for CSS, YAML, JSON, TOML, and Astro; None for all other language identifiers (caller should treat as unsupported).