incremental collaboration sync with targeted bootstrap

[hweihwang]

Problem

Whiteboard collaboration was rebroadcasting too much scene data, and incremental sync bookkeeping could miss or misclassify element updates.

What Changed

• websocket scene sync now sends incremental SCENE_UPDATE payloads when only part of the scene changed
• incremental broadcast markers now track both version and versionNonce
• marker state only advances for elements that were actually transmitted
• the syncer still performs periodic full-scene healing and late-join convergence via full SCENE_INIT rebroadcasts
• pending websocket sync is flushed on unload / hidden / unmount so trailing edits are not dropped
• added integration coverage for incremental bookkeeping and marker updates