Follow-ups from the coordinator session-loop review #3

Non-blocking items carried out of the review of PR #2. None affects correctness of a verdict; all were consciously left rather than missed.

• No test for the conflicting-hash replay refusal. CoordinatorCore.replayRegistered refuses to boot when one session id carries two different testSetHash values. The refusal path has no coverage, and it is a path that only runs at boot — so a regression there is invisible until a real conflicting log appears.

• result() and nack() do not join(). A shard whose lease expires mid-test is marked departed, and un-marks itself on its next claim or register. But a shard actively posting accepted results for its other valid leases still reads as departed in mid-run views until it next claims. Cosmetic — the CI verdict's single post-run GET is unaffected, and PASSED/FAILED ignore the roster entirely — but a join() inside applyResult would close the window.

• Directory fsync throws on Windows. bumpIncarnation does FileChannel.open(dir, READ).force(true) after the atomic move, which is correct on Linux and throws on Windows — and here that propagates into a refused start. Fine for the containerized target; it means the coordinator cannot run on a Windows dev box. A tolerant catch would fix it.

• No saveIfDirty at shutdown. The 30s debounced snapshot flusher is stopped with shutdownNow, discarding a pending flush. Harmless — the snapshot is a boot accelerator and the rows are already in the history log — but a shutdown hook would avoid the needless cold fold on next start.

• Maintenance task uses Clock.systemUTC() directly rather than the injected clock. Testability only.