Phase 5 Stage D wall: port CurveWallTool to DragAction (first wall D)

Direct copy of the fence curve recipe — pure
`curveWallDragAction` (chord-perpendicular projection + clamp +
normalize + single-undo dance) plus a thin wrapper feeding
`useDragAction`. Mounted via `def.affordanceTools.curve`.

Slight precision difference vs legacy: the legacy CurveWallTool snapped
the pointer position to `getWallGridStep()` before projecting onto the
chord normal; the ported action skips that pre-snap and relies on
`normalizeWallCurveOffset` to settle the final value. The user-visible
result is the same magnitude of step, just with the snap applied at
the offset level instead of the position level.

Remaining wall D affordances (endpoint move, whole-wall move,
placement) are larger and queued for future sessions — wall's move
tool alone is 804 LoC with the linked-wall corner-cascade logic.

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

packages/nodes/src/wall/definition.ts

@@ -57,6 +57,14 @@ export const wallDefinition: NodeDefinition<typeof WallNode> = {
 
   parametrics: wallParametrics,
 
+  // Stage D (in progress): drag-affordance components owned by the kind.
+  // Only the curve affordance is ported today — the remaining wall
+  // tools (endpoint drag, whole-wall move, placement) are larger and
+  // queued for future sessions.
+  affordanceTools: {
+    curve: () => import('./curve-tool'),
+  },
+
   renderer: {
     kind: 'parametric',
     module: () => import('./renderer'),