fix(tabletop): align cards to the stacking curve

Fix the tangent angle at the curve's start and rotate cards to follow
the curve direction. Add a stacking curve visualization shown with
surface debug.

packages/tabletop/src/placement.tsx

@@ -28,7 +28,11 @@ export function PartPlacement({ pkg, placement }: { pkg: Package; placement: Pla
   const anchorX = ((candidate?.x ?? route.x ?? 0) + x) * MM_TO_WORLD;
   const anchorY = ((candidate?.y ?? route.y ?? 0) + y) * MM_TO_WORLD;
   const anchorZ = z * MM_TO_WORLD;
-  const anchorRotation = ((candidate?.rotation ?? route.rotation ?? 0) + rotation) * DEG_TO_RAD;
+  // The curve tangent `rotation` is the direction of travel; the card's long
+  // edge ends up along (sinR, cosR) in the surface plane, so it aligns with
+  // the tangent (cosφ, sinφ) when R = 90° − φ.
+  const anchorRotation =
+    ((candidate?.rotation ?? route.rotation ?? 0) - rotation) * DEG_TO_RAD;
 
   return (
     <group position={[anchorX, anchorZ, anchorY]} rotation={[0, anchorRotation, 0]}>