Fix z-ordering reset during collision response

Vector3d.set(x, y) defaults z to 0, which wiped out the z position
used for layer ordering when resolving collisions. Pass the existing
z value explicitly to preserve draw order.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: obiot

packages/melonjs/src/physics/body.js

@@ -477,6 +477,7 @@ export default class Body {
 		this.ancestor.pos.set(
 			this.ancestor.pos.x - overlap.x * ratio,
 			this.ancestor.pos.y - overlap.y * ratio,
+			this.ancestor.pos.z,
 		);
 
 		// cancel the velocity component along the collision normal

packages/melonjs/src/physics/detector.js

@@ -195,6 +195,7 @@ class Detector {
 									objA.body.ancestor.pos.set(
 										objA.body.ancestor.pos.x - overlap.x * ratioA,
 										objA.body.ancestor.pos.y - overlap.y * ratioA,
+										objA.body.ancestor.pos.z,
 									);
 									// cancel velocity into this surface (no bounce)
 									const projVel =
@@ -208,6 +209,7 @@ class Detector {
 									objB.body.ancestor.pos.set(
 										objB.body.ancestor.pos.x + overlap.x * ratioB,
 										objB.body.ancestor.pos.y + overlap.y * ratioB,
+										objB.body.ancestor.pos.z,
 									);
 									const projVel =
 										objB.body.vel.x * overlapN.x + objB.body.vel.y * overlapN.y;