Follow-up on the salvaged pair: the original guard's `not msg_id` arm let an
id-less internal/synthetic event erase a tracking entry a concurrently-queued
id-bearing message's drain task still needs for recall matching (id-less
events never write entries in _dispatch_inbound_event, so they must never
pop). Tests cover: normal cleanup, id-less non-erasure, overwritten-entry
ownership handoff, TTL eviction + fresh-entry survival.
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