feat(editor): placement & interaction overhaul — FSM spine, bug tracks, perf

Implements plans/editor-placement-interaction-overhaul.md: an authoritative
interaction-scope state machine plus the catalogued placement/interaction
fixes, and split-view floor-plan performance.

- Interaction-scope spine (lib/interaction/* + store/use-interaction-scope),
  driven from central useEditor setters; overlay scoping (zone labels,
  context badges, floating action menu) reads resolveOverlayPolicy.
- Bug tracks A/B/D/E/F/G/H: handle/cutout raycast, footprint validity,
  auto-slab loop, ceiling hosting, B-key tool desync, 2D drop offset,
  per-frame jank.
- Snapping modes (grid/lines/angles/off) + contextual HUD chips; modifier
  model (Shift=cycle, Alt=free place, Ctrl=grid step).
- Item move now tracks the cursor 1:1 (was a laggy per-frame lerp); handle
  rig hides during a whole-node move; rotate gizmo advertises Shift=free
  rotation in the HUD and hides the move cross while rotating.
- Floor-plan perf: pause live reactivity while in 3D-only view; per-node
  geometry cache so only changed nodes rebuild on a drag; hoist wall miters
  to a once-per-pass ctx.levelData (O(N^2) -> O(N) on wall/opening drags).

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

packages/nodes/src/slab/move-tool.tsx

@@ -18,6 +18,7 @@ import {
   CursorSphere,
   consumePlacementDragRelease,
   getSegmentGridStep,
+  isMagneticSnapActive,
   markToolCancelConsumed,
   resolveAlignmentForActiveBuilding,
   snapBuildingLocalToWorldGrid,
@@ -174,6 +175,7 @@ export const MoveSlabTool: React.FC<{ node: SlabNode }> = ({ node }) => {
         walls: levelWalls,
         fences: levelFences,
         bypassSnap,
+        magnetic: !bypassSnap && isMagneticSnapActive(),
         gridSnap: (p) => snapBuildingLocalToWorldGrid(p, gridStep),
       })