Phase 5 Stage D fence: port MoveFenceEndpointTool to DragAction

Second Stage D affordance port (425 LoC legacy → ~430 split across
action + wrapper). All math (snap, linked-fence cascade, alt-detach,
min-length gate, single-undo dance) lives in the pure
`moveFenceEndpointDragAction`. The React wrapper handles the UI
overlays (cursor sphere, Drag/Detach badge, angle label) and the live
state subscriptions.

The action introduces the single-undo dance pattern for multi-write
commits: `commit()` calls `scene.restoreAll()` → `resumeHistory()` →
re-applies the final draft so zundo records the entire drag as one
undo step. Reusable shape for slab/wall/door endpoint ports.

Transitional exports added to `@pascal-app/editor`'s public surface
(`snapFenceDraftPoint`, `isWallLongEnough`, the segment-angle helpers,
`MovingFenceEndpoint`). Stage F cleanup moves these into
`@pascal-app/nodes` once every consumer is registry-driven.

`getRegistryAffordanceTool` is now generic (`ComponentType<any>`) so
affordances with different prop shapes (`{ node }`, `{ target }`, …)
all dispatch through the same helper.

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

packages/nodes/src/fence/definition.ts

@@ -76,6 +76,10 @@ export const fenceDefinition: NodeDefinition<typeof FenceNode> = {
     // actions/curve.ts; this component is the React wrapper using
     // useDragAction + the cursor visuals.
     curve: () => import('./curve-tool'),
+    // Triggered by useEditor.movingFenceEndpoint. Pure logic in
+    // actions/move-endpoint.ts (linked-fence cascade, alt-detach,
+    // single-undo dance). Wrapper owns the angle label + detach badge.
+    'move-endpoint': () => import('./move-endpoint-tool'),
   },
 
   toolHints: [