Function scan_with_ref_spec 

async fn scan_with_ref_spec(
    cwd: &Path,
    ref_spec: &str,
    budget: Duration,
) -> ScanOutcome

Scan a pre-resolved diff range into per-file hunks. ref_spec is already the exact shape git receives - HEAD, main...HEAD, <sha>^..<sha>, an empty-tree range, etc. Never surfaces untracked files, so untracked_suppressed is always 0.