Function semantic_diff 

pub fn semantic_diff(
    project_root: &Path,
    old_ref: &str,
    new_ref: &str,
    registry: &LanguageRegistry,
) -> Result<SymbolDiff>

Compute a symbol-level diff between old_ref and new_ref in project_root.

Uses git archive to extract each ref into a temp directory so no working-tree modifications are needed.