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Defense-in-depth hardening of store.sh cmd_enqueue surfaced by the #915 review (obs#2): the final observed_seq cursor _atomic_write was the ONE durable write not wrapped in a failure check, and was cross-file non-atomic with the observed.set write immediately before it. Practically unreachable today, but a cursor-write failure after a successful observed.set write could (a) return spurious success while the allocation stayed uncommitted, and (b) leave observed.set advanced while the cursor lagged (cross-file inconsistent). Fix (additive; preserves #908 exactly): - GATE the final cursor write like the pending/observed.set writes (#908): a cursor-write failure is now FAIL-LOUD (non-zero + diagnostic), never swallowed. - On cursor-write failure, ROLL observed.set BACK to its pre-write snapshot, so observed.set and the cursor either BOTH advance or NEITHER does — never a stranded observed.set entry above the cursor. #908 invariants intact: single store-side allocator, atomic allocate+enqueue under flock, arrow-1 no-burn (pending write still FIRST, still aborts before any cursor advance), anti-swallow <=consumed fail-loud, W2 contiguous-prefix CONSUMED. The cursor stays the sole COMMIT point, so a pending-ahead state is exactly the one #908 already tolerates on its observed.set-failure path. On-disk format UNCHANGED (read-compatible). Test: new T11 in test-wake-store-ack.sh forces the final cursor _atomic_write to fail (bind-mount EBUSY over observed_seq inside an unshare mount+user namespace, seeded so the cursor READ still succeeds) after pending+observed.set succeed, and asserts fail-loud + observed.set/cursor consistency + intact consumed prefix. RED against pre-#917 code; all store-ack/race/reconcile/detector groups stay GREEN. manifest bumped 0.6.8 -> 0.6.9. Closes #917 Part of #892 Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_0158NZqN2n2ymKFeJAZ4GUCb