fix: correct 2D cross product formula in MathUtils

The cross product formula was incorrect:
- Before: p1x * p2x - p1y * p2y (wrong)
- After: p1x * p2y - p1y * p2x (correct)

The 2D cross product (perp dot product) should compute:
A × B = Ax * By - Ay * Bx

This returns the z-component of the 3D cross product when
treating 2D vectors as 3D vectors with z=0.

Also fixed the example comment to show correct result.

Author: arif741

src/utils/MathUtils.ts

@@ -56,7 +56,7 @@ export const lerp = mix;
  * @param p2y 第二个向量的y分量
  * @returns 叉积结果 (标量)
  * @example
- * cross(1, 0, 0, 1) // 返回 0
+ * cross(1, 0, 0, 1) // 返回 1
  */
 export function cross(
   p1x: number,
@@ -68,7 +68,7 @@ export function cross(
     if (!isNumber(p1x) || !isNumber(p1y) || !isNumber(p2x) || !isNumber(p2y)) {
       throw new Error("Invalid parameters for cross function");
     }
-    return p1x * p2x - p1y * p2y;
+    return p1x * p2y - p1y * p2x;
   } catch (error) {
     console.error("Error in cross function:", error);
     return 0;