Function verify_contracts 

pub fn verify_contracts(
    root: &Path,
    file_paths: &[String],
    registry: &ParserRegistry,
    target_file: Option<&str>,
) -> Vec<ContractViolation>

Verify function call contracts across the codebase.

For each Calls edge in the graph, extracts expected param count from the callee’s first line and actual arg count from the call site in the caller’s content. Flags mismatches.

If target_file is Some, only report violations for callees in that file.