Phase 5 Stage D: fix bend history + move-tool loop + grid-snap sfx

Three user-reported regressions, all in the Stage D move/curve ports:

1. **Fence/wall bend cancelled the fence creation on Ctrl-Z.** The
   action.commit's `return false` shortcut on "no offset change"
   bypassed the dance entirely — pastStates wasn't touched, so the
   next Ctrl-Z fell through to whatever preceded activation
   (typically the create step). Fix: always run the dance, even on
   no-op commits. First Ctrl-Z then absorbs a silent no-op entry,
   subsequent presses roll back real prior actions. Same fix applied
   to fence move-endpoint, fence move, slab move, ceiling move.

2. **Slab/ceiling move 'maximum update depth exceeded' loop.** The
   `useScene` selector in `SlabMoveTool` returned a freshly-allocated
   `[sx, sz]` tuple on every call. Zustand's `Object.is` equality
   failed each comparison → re-subscribe → re-render → loop. Fix:
   subscribe to the stable live-node reference and derive the center
   via `useMemo`. Same recipe for fence/ceiling move-tools, including
   memoizing the `originalCenter` fallback that was getting a new
   array per render.

3. **No grid-snap sfx during move drag.** Action `preview` now tracks
   the last snapped pointer on a mutable `lastSnapped` ctx field and
   emits `sfx:grid-snap` when it changes between ticks. Matches the
   legacy MoveFenceTool's per-tick sound.

Locking test for foot-gun 1 lives in
`packages/core/src/services/single-undo-dance.test.ts`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/nodes/src/slab/actions/move.ts

@@ -8,7 +8,11 @@ import {
   useScene,
   type WallNode,
 } from '@pascal-app/core'
-import { type FencePlanPoint, snapFenceDraftPoint } from '@pascal-app/editor'
+import { type FencePlanPoint, snapFenceDraftPoint, triggerSFX } from '@pascal-app/editor'
+
+function sameSnap(a: FencePlanPoint | null, b: FencePlanPoint): boolean {
+  return a !== null && a[0] === b[0] && a[1] === b[1]
+}
 
 /**
  * Phase 5 Stage D — whole-slab move drag affordance.
@@ -16,7 +20,8 @@ import { type FencePlanPoint, snapFenceDraftPoint } from '@pascal-app/editor'
  * Translates the slab's boundary polygon (and any holes) rigidly under
  * the pointer. Snaps to walls / fences / grid at the level. Latches
  * the drag anchor on the first preview tick so the slab doesn't jump
- * to wherever the activation click landed.
+ * to wherever the activation click landed. Emits grid-snap sfx when
+ * the snapped position changes between ticks (matches legacy UX).
  *
  * Unlike fence move, the slab port does **not** use the live-drag
  * exception — polygon CSG geometry is expensive to rebuild per frame,
@@ -52,6 +57,7 @@ export type MoveSlabCtx = {
   levelWalls: WallNode[]
   levelFences: FenceNode[]
   dragAnchor: FencePlanPoint | null
+  lastSnapped: FencePlanPoint | null
 }
 
 export type MoveSlabDraft = {
@@ -98,6 +104,7 @@ export const moveSlabDragAction: DragAction<MoveSlabCtx, MoveSlabDraft> = {
       levelWalls,
       levelFences,
       dragAnchor: null,
+      lastSnapped: null,
     }
   },
 
@@ -107,6 +114,11 @@ export const moveSlabDragAction: DragAction<MoveSlabCtx, MoveSlabDraft> = {
       walls: ctx.levelWalls,
       fences: ctx.levelFences,
     })
+    // Emit grid-snap sfx when the snapped position changes.
+    if (!sameSnap(ctx.lastSnapped, snapped)) {
+      if (ctx.lastSnapped !== null) triggerSFX('sfx:grid-snap')
+      ctx.lastSnapped = snapped
+    }
     if (!ctx.dragAnchor) ctx.dragAnchor = snapped
     const deltaX = snapped[0] - ctx.dragAnchor[0]
     const deltaZ = snapped[1] - ctx.dragAnchor[1]
@@ -130,11 +142,9 @@ export const moveSlabDragAction: DragAction<MoveSlabCtx, MoveSlabDraft> = {
   },
 
   commit: (draft, ctx, scene) => {
-    if (draft.deltaX === 0 && draft.deltaZ === 0) return false
-
-    // Single-undo dance — revert via snapshot, resume history, re-apply
-    // the final polygon/holes. Zundo captures the whole drag as one
-    // Ctrl-Z step.
+    // Always push — see fence/actions/curve.ts. Even on a no-movement
+    // commit, the dance must push a pastState entry so Ctrl-Z doesn't
+    // cancel whatever was on the stack before activation.
     scene.restoreAll()
     scene.resumeHistory()
     scene.update(ctx.slabId, {