Advances lifecycle status to Dismissed (dismissalSource=UserDismissed, stamps events.dismissedAt, records actors.dismissedBy, fires Notification.Dismissed). A NonDismissable notification returns 409 InvalidStateTransition (not a 400). Person-scoped. The inbox is single-owner and view/dismiss transitions are monotonic and idempotent (Active to Viewed to Dismissed); concurrent writers cannot produce a lost-update anomaly, so ETag/If-Match is intentionally omitted and no version field is exposed. Replaying an Idempotency-Key with a different body returns 409 IdempotencyKeyConflict.