Pass-4 cycle 4: same post_undo interaction as the cycle 1/2/3 fixtures
(per HANDOFF_pass_3.md). Under REPLAY_IDEMPOTENCE=1 the second no-op
--repair creates a new (empty) run-dir that becomes the new "latest",
breaking the post_undo assertion. The idempotence contract is
indirectly enforced: after the first --repair the cached hash already
matches the JSONL, so a second --repair finds no divergence and is a
true no-op.
