feat(editor): roof/stair/elevator snapping migration + no-angle footprint draft

Migrate roof/stair/elevator draft tools + the 2D floorplan move overlay off the
legacy Shift/Alt=bypass model onto mode-driven snapping (isGridSnapActive /
isMagneticSnapActive); Alt dropped (no validity gate). stair/elevator now use the
live grid step.

These three are placed as footprints, not directional draws, so the angle-lock
mode was meaningless: add NodeDefinition.snapDraftDirectional (default true; false
for roof/stair/elevator) so their draft resolves to the no-angle 'polygon' context
(grid / lines / off). snapContextOf takes an injected draftDirectionalOf, like
profileOf. Add toolHints to stair/elevator so they route through the contextual
HUD and show the snapping chip. Fix one stale Alt-bypass comment in the item
placement coordinator (#9: already force-only). +snapping-mode test.

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

packages/core/src/registry/types.ts

@@ -986,6 +986,17 @@ export type NodeDefinition<S extends ZodObject<any>> = {
    */
   snapProfile?: SnapProfile
 
+  /**
+   * For `structural` kinds: does drafting this kind set a DIRECTION (so the
+   * angle-lock snapping mode is meaningful)? Wall/fence/slab/ceiling drafting
+   * draws directed edges → `true` (the default). Roof/stair/elevator are placed
+   * as axis-aligned footprints, not directional draws → `false`, so their
+   * drafting uses the no-angle `polygon` snap context (grid / lines / off)
+   * instead of the angle-bearing `wall` context. Ignored for `item` kinds
+   * (their context never carries an angle lock).
+   */
+  snapDraftDirectional?: boolean
+
   /**
    * Optional translucent preview of the node — used by the move tool to
    * show where the node will land, and by the placement tool's cursor.