On a case-insensitive filesystem (Windows, macOS) `scripts/check --staged`/`--commit`
and the hooks' judge() aborted with "checkout-index -a failed: pkg/Notes.md already
exists" when the judged tree held two paths that differ only by case, so the
case-collision guard never got to report them (exit 2 instead of FAILED:
case-collisions). Both snapshots now check out with -f into their empty temp dir; the
guards read the private index, which keeps both paths. Found on the Windows runner by
the case-collision test once that test staged its premise as blobs.
bdef06de3b