Pass-4 cycle 3: same post_undo interaction as cycles 1, 2, and the
gitignore fixtures (per HANDOFF_pass_3.md). The post_undo stage
asserts that `undo latest` restores the FIRST repair; under
REPLAY_IDEMPOTENCE=1 the second no-op --repair creates a newer
(empty) run-dir that becomes the new "latest", breaking post_undo.

The idempotence contract is still indirectly enforced: a second
--repair against a clean (post-quarantine) workspace finds no aged
artifacts and emits zero actions.
