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/stair/definition.ts

@@ -422,6 +422,10 @@ export const stairDefinition: NodeDefinition<typeof StairNode> = {
   schema: StairNode,
   category: 'structure',
   snapProfile: 'structural',
+  // A footprint with a clear front: you approach a stair from the low end,
+  // which sits on the -Z side of the run (the run ascends along +Z). Show the
+  // floor facing triangle there, pointing out of the entry, while placing/moving.
+  facingIndicator: { reversed: true },
   // Placed as a footprint (R/T rotates), not a directional draw → no angle-lock
   // mode. The toolHints presence routes it through the contextual HUD so the
   // snapping chip shows during placement.