A shell-launched 'hermes gateway run --replace' / 'gateway restart' on a
systemd/launchd host can leave an orphan gateway whose kanban dispatcher
escapes the service cgroup, survives 'systemctl restart', and becomes a
second long-lived writer on the shared kanban.db. Two dispatchers that each
believe they own the file both pass SQLite busy_timeout and then race on WAL
frames — the documented root cause of multi-writer corruption (issue #35240).
The existing _guard_supervised_gateway_conflict startup guard blocks the
common way an orphan is born, but does nothing once a second dispatcher
already exists. This adds the defense-in-depth: dispatch_once now wraps every
tick in a non-blocking, board-scoped flock (_dispatch_tick_lock). A losing
dispatcher returns DispatchResult(skipped_locked=True) and does zero DB writes
this tick — so two dispatchers can never run a reclaim/spawn/write sequence
concurrently regardless of how the second one got there.
- Non-blocking (LOCK_NB): never stalls the gateway's async watcher.
- Board-scoped: lock file is a .dispatch.lock sibling of each board's
kanban.db, so unrelated boards tick in parallel.
- POSIX + Windows (fcntl / msvcrt LK_NBLCK), no-op degrade where neither
exists — mirrors the existing _cross_process_init_lock pattern.
Verified with a real two-process orphan repro: while a separate process holds
the lock, dispatch_once skips; after release it runs.
