Collab: Pause incoming remote changes while CJK composition is active

## Steps to reproduce

1. In the first Paragraph block, Peer 1 types in English.
2. In the second Paragraph block, Peer 2 simultaneously types in Japanese using an IME.
3. Peer 2 will have trouble entering characters because their input gets overwritten by remote updates before IME composition is finalized.

## Possible fix

Listen for [`compositionstart`](https://developer.mozilla.org/en-US/docs/Web/API/Element/compositionstart_event) events and pause the processing of incoming remote changes until the `compositionend` event is detected.

Start by tracing the incoming remote-change handling and the editor's compositionstart/compositionend event flow. Reproduce the two-paragraph collaboration scenario with Japanese IME input; done means remote changes no longer overwrite text while composition is active and resume after compositionend.

Written by the indexing model from the issue text.