session_focus_already_committed· reason_code· applies to event_envelopeA subsequent `ak.call.state` event attempted to write a `session_focus` value different from the already-committed one. The reducer MUST `failed_precondition` — `session_focus` is write-once per call lifecycle; in-session focus migration is not supported in v1. See zh/crypto-media/call-state.md §4.1.