feat(editor): unify placement/move facing triangle into one editor-side renderer

Every placement and move path now publishes its ghost pose to a single
`useFacingPose` store, drawn by one editor-side `<FacingPoseIndicator>` overlay,
instead of each path drawing its own triangle (which left the nodes-package and
PlacementBox paths invisible):

- column/shelf presets + all moves (PlacementBox via move-registry, and
  DragBoundingBox) now publish the pose, so the triangle finally shows
- stair create + move use a declarative `facingIndicator: { reversed: true }`
  (new registry resolver) so the triangle sits before the entry pointing out —
  resolved in one place, so create and move match automatically
- stair placement defaults to single and respects the shared `point`
  continuation (C) toggle, like the other placement tools

Checkpoint on the placement-interaction epic: also carries the in-flight
continuation-profile extraction (lib/continuation), grid surface (item #8), and
HUD work. Door/window still render their own legacy inline triangle and are
migrated to the overlay next.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/nodes/src/shelf/definition.ts

@@ -133,6 +133,7 @@ function shelfHandles(_node: ShelfNodeType): HandleDescriptor<ShelfNodeType>[] {
 export const shelfDefinition: NodeDefinition<typeof ShelfNode> = {
   kind: 'shelf',
   snapProfile: 'item',
+  facingIndicator: true,
   schemaVersion: 2,
   schema: ShelfNode,
   category: 'furnish',

packages/nodes/src/shelf/tool.tsx

@@ -128,10 +128,15 @@ const ShelfTool = () => {
       useScene.getState().createNode(shelf, activeLevelId)
       useViewer.getState().setSelection({ selectedIds: [shelf.id] })
       triggerSFX('sfx:item-place')
-      // The placed shelf is now a valid alignment target for the next one;
-      // refresh the candidate pool and drop the guide from this drop.
-      alignmentCandidates = collectAlignmentAnchors(useScene.getState().nodes, previewNode.id)
       useAlignmentGuides.getState().clear()
+      if (useEditor.getState().getContinuation('point') === 'repeat') {
+        // The placed shelf is now a valid alignment target for the next one.
+        alignmentCandidates = collectAlignmentAnchors(useScene.getState().nodes, previewNode.id)
+      } else {
+        cursorVisibleRef.current = false
+        setCursorVisible(false)
+        useEditor.getState().setTool(null)
+      }
 
       stopPlacementCommitPropagation(event)
     }