Phase 5 Stage D: fence curve affordance → registry DragAction

First Stage D port — `CurveFenceTool` (178 LoC legacy) split into a pure
`DragAction` primitive (`packages/nodes/src/fence/actions/curve.ts`) plus
a thin React wrapper (`packages/nodes/src/fence/curve-tool.tsx`) that
feeds it through `useDragAction`. The kind declares the affordance via
`def.affordanceTools.curve`; ToolManager lazy-loads it at runtime when
`useEditor.curvingFence` activates. Falls back to the legacy
`CurveFenceTool` for any kind that hasn't been ported.

The lazy-load dispatch dodges the editor→nodes circular dep (nodes
already depends on editor for `useDragAction` + `CursorSphere`).

Establishes the pattern for the remaining fence affordances
(`MoveFenceEndpoint`, `MoveFence`, placement) and for slab/ceiling/wall
D ports.

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

packages/nodes/src/fence/definition.ts

@@ -67,6 +67,16 @@ export const fenceDefinition: NodeDefinition<typeof FenceNode> = {
   // Legacy `floorplanFenceEntries` short-circuits to [] when fence is
   // registered (see floorplan-panel.tsx).
   floorplan: buildFenceFloorplan,
+  // Stage D (in progress): drag-affordance components owned by the kind.
+  // ToolManager looks up these lazy modules at runtime when the matching
+  // editor state activates — no static import from editor → nodes
+  // (which would create a circular dep).
+  affordanceTools: {
+    // Triggered by useEditor.curvingFence. Pure DragAction logic in
+    // actions/curve.ts; this component is the React wrapper using
+    // useDragAction + the cursor visuals.
+    curve: () => import('./curve-tool'),
+  },
 
   toolHints: [
     { key: 'Left click', label: 'Set fence start / end' },