Phase 5 Stage D moves: live-drag mesh.position for slab / ceiling / fence

User-reported regressions on the 1:1 legacy ports: slab/ceiling moves
were slow (polygon CSG rebuilds per scene.update tick), fence moves
teleported briefly on commit (residual mesh.position offset survived
the geometry rebuild). All three now use the same live-drag pattern
the legacy fence move was designed for:

- During drag, write only to `sceneRegistry.nodes.get(id).position`
  + `useLiveTransforms`. No `scene.update`, no polygon rebuild, no
  React re-render of geometry.
- History stays UNPAUSED — scene state isn't changing.
- On commit, a single `scene.update` writes the translated
  polygon (or fence start/end + linked-fence cascade). Recorded as one
  natural undo step.
- Tools leave `mesh.position` at the drag delta on commit;
  GeometrySystem / CeilingSystem reset it to (0,0,0) when they
  rebuild the geometry on the next frame. By the time position
  clears, the new geometry is in place — no teleport.

Two framework changes enable this:
- `GeometrySystem` (viewer/systems/geometry) now resets
  `group.position` + `group.rotation` after every rebuild, matching
  the legacy `FenceSystem.updateFenceGeometry` behavior. Tools that
  translate the group during live-drag can rely on the reset.
- Legacy `CeilingSystem.updateCeilingGeometry` extends its existing
  `position.y` reset to cover X/Z too — previously it left X/Z at the
  drag delta after rebuild, double-translating the visual.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/viewer/src/systems/ceiling/ceiling-system.tsx

@@ -54,7 +54,13 @@ function updateCeilingGeometry(node: CeilingNode, mesh: THREE.Mesh) {
     gridMesh.geometry = newGeo.clone()
   }
 
-  // Position at the ceiling height
+  // Position at the ceiling height and reset X/Z so live-drag mesh
+  // offsets (set by move tools during the drag) don't leak into the
+  // canonical position after the rebuild. Matches the pattern used by
+  // FenceSystem.updateFenceGeometry / GeometrySystem (both fully reset
+  // position+rotation after rebuild).
+  mesh.position.x = 0
+  mesh.position.z = 0
   mesh.position.y = (node.height ?? 2.5) - 0.01 // Slight offset to avoid z-fighting with upper-level slabs
 }