Merge origin/main into feat/placement-interaction-overhaul

Resolve 7 conflicts keeping our snapping migration + floorplan perf work as
source of truth, combined with main's MEP run-continuation / Alt-detach /
latch handles. Rebuilt two import blocks the auto-merge silently truncated
(node-arrow-handles.tsx, duct-fitting/move-tool.tsx).

Verified: tsc clean across core/viewer/editor/nodes/mcp, 451 tests pass,
biome clean. Floorplan view-transform re-render storm confirmed pre-existing
(not introduced by this merge).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/nodes/src/fence/tool.tsx

@@ -560,6 +560,7 @@ export const FenceTool: React.FC = () => {
     }
 
     const onGridClick = (event: GridEvent) => {
+      if (!previewRef.current) return
       if (buildingState.current === 1 && event.nativeEvent.detail >= 2) {
         stopDrafting()
         return