Recall continuation usability gate

Never execute recall or accept recovery from an incomplete principal or unusable continuation. Recovery requires a protocol-complete envelope with identity, model, usage, assistant role, content, and allowlisted successful terminal semantics; OpenAI Chat accepts exactly one choice and only stop, tool_calls, or length, with length still unusable for recovery. Malformed JSON, failure aliases, blank tools, private reasoning-only output, repeated recall, exhaustion, or cancellation must return the fixed public failure without persistence or partial output.