pub fn build_long_function_report(
root: &Path,
threshold: usize,
explicit_files: Option<&[PathBuf]>,
walk_config: &WalkConfig,
) -> DiagnosticsReportExpand description
Build a DiagnosticsReport for the long-function rule.
Walks all source files under root, parses each with tree-sitter, and emits
an issue for every function whose line span meets or exceeds the threshold.