editor: level-scoped alignment, reference-floor symbols & registry slab tool (#373)

* feat(editor): level-scoped alignment, reference-floor registry symbols, registry slab tool

- Scope alignment candidates to the active level so a node directly below
  on another floor no longer snaps (alignment is XZ-only); building-scoped
  nodes like elevator shafts stay in the pool across floors.
- Derive the elevator alignment footprint from its outer shaft (not the
  inset cab), so its guide actually surfaces within the snap threshold.
- Extract shared stair footprint geometry (rotateXZ, segment transforms,
  stairFootprintAABB) so opening-sync and alignment anchors derive the
  chain identically; stairs now contribute plan bbox anchors.
- Render reference-floor stairs/roofs/elevators/shelves/spawns through
  their registry floorplan builders for pixel-identical symbols.
- Move slab creation to the registry-driven slab tool (parity with
  ceiling); 2D handlers only maintain draft state.
- Lift the 3D alignment guide ribbon to the active level's Y each frame.

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

* refactor(core): registry-drive elevator/stair alignment footprints

Address architecture review of the level-scoped alignment work: the core
anchor bridge (`alignment-anchors.ts`) hardcoded `if (node.type === 'elevator')`
and `if (node.type === 'stair')` branches to derive their plan footprints.
Move that onto the kinds themselves via a new `alignmentFootprint` capability
so the bridge dispatches generically — matching the registry composition model.

- Add `Capabilities.alignmentFootprint`: returns a `box` (rotatable rect
  centred on position, relocatable for movable kinds) or an `aabb` (already
  resolved, for non-rectangular plan shapes). Elevator uses `box` (its outer
  shaft, and it's movable); stair uses `aabb` (segment chain / annular sector,
  moves by origin).
- Drop both hardcoded branches; the bridge now consults the capability via
  `floorFootprint` (box) and a unified `alignmentAABB` (box ∪ aabb).
- Export `stairFootprintAABB` from core so the stair definition consumes it.
- Tests register synthetic defs carrying the capability (the bridge no longer
  knows elevator/stair by name), reproducing the production glue from the same
  core helpers.
- Document why `REFERENCE_REGISTRY_KINDS` is a deliberate editor-local
  curation, not an auto-derived set (most floorplan-builder kinds shouldn't
  appear as standalone reference symbols, and "reference floor" is an editor
  concept core must not know).

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

* fix(editor): hide node arrow handles during thumbnail capture

The arrow-handle rig lives on SCENE_LAYER (so its chevrons read as proper
3D plates), but the thumbnail camera only filters EDITOR_LAYER + GRID_LAYER —
so a node selected at capture time would leak its arrows into the snapshot.
Hide the rig on `thumbnail:before-capture` and restore it on
`thumbnail:after-capture`, the same emitter handshake SelectionManager uses.

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

---------

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

packages/nodes/src/stair/definition.ts

@@ -3,6 +3,7 @@ import {
   type NodeDefinition,
   StairNode as StairNodeSchema,
   type StairNode as StairNodeType,
+  stairFootprintAABB,
 } from '@pascal-app/core'
 
 const MIN_CURVED_RISE = 0.3
@@ -318,6 +319,15 @@ export const stairDefinition: NodeDefinition<typeof StairNode> = {
 
   capabilities: {
     selectable: { hitVolume: 'bbox' },
+    // A stair has no centred box footprint: straight = a cumulative
+    // `stair-segment` chain, curved / spiral = an annular sector. Hand the
+    // alignment bridge the resolved plan `aabb` directly (not a `box`) — the
+    // stair moves by its origin via `affordanceTools.move`, so it only ever
+    // contributes static candidate anchors, never the relocatable box path.
+    alignmentFootprint: (node, nodes) => {
+      const aabb = stairFootprintAABB(node as StairNodeType, nodes)
+      return aabb ? { shape: 'aabb', ...aabb } : null
+    },
     duplicable: true,
     deletable: true,
   },