On NTFS the fixture self-collides: .builtin-manifest auto-generates the
8.3 short name BUILTI~1, so writing that fixture file opens the manifest
through its alias and clobbers it -- the exact threat the guard models.
Gate the fixture-based test to unix, and add a windows counterpart that
proves the collision where 8.3 generation is enabled and asserts the
filter invariant (alias-shaped entries are neither removal candidates
nor claimed) either way.
Dark-Alex-17
bf10aeb74d