SDK stream-json schema gains historical and runner_exit fields, plus expanded meta-turn uuid stamping

What

• Message schema gains an internal historical boolean field, stamped by the Remote Control bridge on messages it flushes to the session server, and by the server on deliveries it replays.
• A new runner_exit field is set by the self-hosted runner when a session process fails to start or exits abnormally.
• Documentation for user_message_uuid/user_message_uuids (across turn, error-result, and thinking_tokens frames) was expanded: a turn started by a synthetic/scheduled (meta) prompt can now stamp a later, non-ping stream event when a queued user message is folded into the turn mid-stream, in addition to the turn's first stream event. A turn started by a typed prompt still stamps only its first event.

Why These are schema and documentation additions for SDK consumers: historical and runner_exit give more visibility into replayed messages and runner failures, and the clarified uuid-stamping rules help SDK clients correctly match stream events to the user message that triggered them, including when a message is folded into an already-running turn.