circle_count_exceeded· reason_code· applies to event_envelope, state_resolutionReducer rejected a `ak.circle.create` exceeding the per-Realm active Circle cap, or a `ak.circle.member.state -> join` that would exceed the per-actor active MLS-backed Circle cap. Caps bound cascade/delivery fanout and the M+R MLS-rotation amplification of a single membership change (zh/conformance/scalability-constraints.md §5, zh/models/circle.md §10).