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/fence/move-tool.tsx

@@ -3,6 +3,7 @@
 import { type FenceNode, useLiveTransforms } from '@pascal-app/core'
 import { CursorSphere, triggerSFX, useDragAction, useEditor } from '@pascal-app/editor'
 import { useViewer } from '@pascal-app/viewer'
+import { useMemo } from 'react'
 import { moveFenceDragAction } from './actions/move'
 
 /**
@@ -10,8 +11,13 @@ import { moveFenceDragAction } from './actions/move'
  *
  * Replaces the legacy `MoveFenceTool` (302 LoC). The action owns all
  * the math (snap + linked cascade + live-drag mesh offsets +
- * single-undo dance on commit). The wrapper just renders the cursor
- * sphere and tracks its position from the live-transform store.
+ * single-undo dance on commit). The wrapper renders the cursor sphere
+ * tracking its position from the live-transform store.
+ *
+ * Selector stability: `originalCenter` is memoized so the live-transform
+ * fallback doesn't return a new array per render — that pattern blows
+ * up zustand's `Object.is` check and trips "getSnapshot result not
+ * cached" → infinite re-render. Same recipe in slab/ceiling move-tool.
  *
  * Mounted by ToolManager when `useEditor.movingNode` is a fence
  * (capability-driven dispatch — fence has no `movable` capability, so
@@ -20,19 +26,19 @@ import { moveFenceDragAction } from './actions/move'
  */
 export const FenceMoveTool: React.FC<{ node: FenceNode }> = ({ node }) => {
   const fenceId = node.id
-  const originalCenter: [number, number, number] = [
-    (node.start[0] + node.end[0]) / 2,
-    0,
-    (node.start[1] + node.end[1]) / 2,
-  ]
+  const originalCenter: [number, number, number] = useMemo(
+    () => [(node.start[0] + node.end[0]) / 2, 0, (node.start[1] + node.end[1]) / 2],
+    [node.start, node.end],
+  )
 
-  // Live position from the live-transforms store — the action writes
-  // here every preview tick (live-drag exception). Falls back to the
-  // original center until the first move.
-  const liveCenter = useLiveTransforms((s) => {
-    const t = s.get(fenceId)
-    return t?.position ?? originalCenter
-  })
+  // Subscribe to the live-transform reference only (stable across
+  // renders unless set/clear was called). Derive position via useMemo
+  // so the selector itself stays cached.
+  const liveTransform = useLiveTransforms((s) => s.get(fenceId))
+  const liveCenter: [number, number, number] = useMemo(
+    () => liveTransform?.position ?? originalCenter,
+    [liveTransform, originalCenter],
+  )
 
   const exitMoveMode = (committed: boolean) => {
     if (committed) triggerSFX('sfx:item-place')
@@ -53,7 +59,7 @@ export const FenceMoveTool: React.FC<{ node: FenceNode }> = ({ node }) => {
 
   return (
     <group>
-      <CursorSphere position={liveCenter as [number, number, number]} showTooltip={false} />
+      <CursorSphere position={liveCenter} showTooltip={false} />
     </group>
   )
 }