fix(client): correctly handle coordinate SceneUI attachment

[AnrokX]

Summary

• Mark coordinate-based SceneUI instances for an initial position sync when they are created on the client.
• Clear any prior entity attachment when SceneUI.setPosition(...) is used, keeping attachment modes mutually exclusive.

Notes

• Added a follow-up PR comment in the fork PR describing a pending-template edge case and concise fix direction.

Testing

• Manual local validation from the original PR:
  • SceneUI renders and remains hooked to the configured world coordinate.

[AnrokX]

Edge case: while SceneUI is pending template registration, merge logic drops undefined fields. That can lose the detach signal (attachedToEntityId = undefined) when switching to position, so it may still instantiate in entity-attached mode.

Minimal fix: in pending merge, treat position and attachedToEntityId as mutually exclusive and clear the opposite field when one is set.