session_focus_no_split_brain· reason_code· applies to service_callThe committed `ak.call.state.session_focus` is authoritative and write-once: once it exists, a connect / token-exchange failure against that focus MUST be surfaced as focus-unavailable (`focus_unavailable_for_client`) and clients MUST NOT silently fall back to a different focus to keep the media path up. Naming the invariant explicitly closes the split-brain attack where two subsets of a conference converge on different SFUs. See zh/crypto-media/media-service-binding.md §5 and §2 (`foci[].health_endpoint`).