## Why
Clients need to refresh section membership and ordering when another client moves a thread.
## What changed
Broadcast `thread/section/changed` after a successful `thread/section/move`, including moves within the same section. Send the response before the notification, and include `threadId` and the destination `sectionId`, using `null` for removal. Failed moves do not emit the notification.
Add the notification to the protocol schemas, TypeScript exports, and Python SDK models and registry, and document its behavior.
## Testing
Add a WebSocket integration test with separate writer and observer clients covering section insertion, reordering, removal, and a failed move.
GitOrigin-RevId: 128ba7eddc9aae8e3bac766f47e4a2c55c64a6a5
