3 RED carriers (duplicate FINISH on DONE appends a 2nd WAL entry +
overwrites state; START on terminal clobbers it; replay non-equivalent)
+ 2 preservation pins (forward postdone PDONE-merge, fail-after-finish
PERR-merge) that 6.2 must not break. Reviewer-confirmed: the
UPDATE-clears-terminal arm is production-unreachable (pinned, not
driven). 22 tests / 3 RED; census 508 distinct triples, only those 3
fail.