## What changed
Add the experimental `thread/readState/changed` notification with `threadId` and the committed `readState` receipt. Send it to existing thread subscribers when terminal activity updates read state or a revert clears a removed unread position. Emit terminal read-state updates before `turn/completed` so completion remains the last notification for that result.
Expose the notification in generated JSON schemas, TypeScript types, and Python SDK models. Document experimental capability gating, subscription lifetime, notification opt-out, and refreshing receipts when concurrent responses disagree.
## Testing
Extend the read-state integration test to assert that the notification receipt matches the receipt returned by `thread/read` after terminal activity.
GitOrigin-RevId: 28935352a881b7a5e2e7f629173c3b2061bd7941
