initSpaceDetectionSync subscribes to every scene mutation and auto-derives
slabs/ceilings from wall topology. When a host app drives explicit
slab/ceiling creation (e.g. the community editor's AI create_room flow),
the auto-sync races those nodes and the polygon-signature de-dupe is
fragile enough that duplicates leak through.
Expose pauseSpaceDetection / resumeSpaceDetection / isSpaceDetectionPaused
mirroring the existing pauseSceneHistory refcount in store/history-control.
While paused, the subscriber rolls previousSnapshots forward so resume
does NOT trigger a backfill that would re-introduce the very duplicates
the host paused to avoid.
No behavior change for callers that don't touch the new functions.
