Function commit_count_in_range 

async fn commit_count_in_range(
    cwd: &Path,
    base: &str,
    head: &str,
    budget: Duration,
) -> Option<u32>

Count commits in <base>..HEAD via git rev-list --count. Returns None on any failure path (subprocess error, parse failure, anomalous empty output) so callers can route the signal into LayerState::ScanFailed instead of masking the failure as 0. Each non-Ok branch emits a WARN log mirroring the gh_pr_lookup_* pattern.