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/item/definition.ts

@@ -315,7 +315,8 @@ export const itemDefinition: NodeDefinition<typeof ItemNode> = {
     { key: 'Left click', label: 'Place item' },
     { key: 'R', label: 'Rotate counterclockwise' },
     { key: 'T', label: 'Rotate clockwise' },
-    { key: 'Shift', label: 'Free place' },
+    { key: 'Shift', label: 'Cycle snapping mode' },
+    { key: 'Alt', label: 'Free place (no snap)' },
     { key: 'Esc', label: 'Cancel' },
   ],
 

packages/nodes/src/item/floorplan.ts

@@ -182,6 +182,7 @@ export function buildItemFloorplan(node: ItemNode, ctx: GeometryContext): Floorp
   })
 
   const isSelected = ctx.viewState?.selected ?? false
+  const isMoving = ctx.viewState?.moving ?? false
   const floorPlanUrl = node.asset.floorPlanUrl
   const children: FloorplanGeometry[] = [
     {
@@ -214,8 +215,10 @@ export function buildItemFloorplan(node: ItemNode, ctx: GeometryContext): Floorp
       rotation: transform.rotation,
     })
   }
-  // Move handle — orange dot at the item center. Only when selected.
-  if (isSelected) {
+  // Move handle — orange dot at the item center. Only when selected and not
+  // already moving: during a move the dot sits under the cursor, so a release
+  // over it would re-arm the move (and re-enter edit) instead of committing.
+  if (isSelected && !isMoving) {
     children.push({
       kind: 'move-handle',
       point: [cx, cy],