core(space-detection): add pause/resume refcount primitive (#336)

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.

packages/core/src/index.ts

@@ -56,7 +56,10 @@ export {
   type AutoSlabSyncPlan,
   detectSpacesForLevel,
   initSpaceDetectionSync,
+  isSpaceDetectionPaused,
+  pauseSpaceDetection,
   planAutoSlabsForLevel,
+  resumeSpaceDetection,
   type Space,
   wallTouchesOthers,
 } from './lib/space-detection'