HashSet<T>.First() enumeration order is unspecified and unstable across mutations, so
the "owner" handle attached to alarm events was non-deterministic when multiple alarm
subscriptions were active. Change _alarmSubscriptions from HashSet to List (preserving
insertion order) and pick [0] — the earliest-registered handle — as the deterministic
owner. The server routes transitions by SourceNodeId, not by handle, so the choice of
handle does not affect delivery to active subscribers.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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