Split overworldscene.js's player/plane/camera into their own classes

Extracts Player (extends Entity), TestPlane, and PlayerCamera into
their own files; the camera now tracks the player's live position
at a fixed offset instead of orbiting the world origin over time.
Updates test_overworldscene.c to match the new entity order and to
build an in-memory zip fixture, since overworldscene.js now
require()s these sibling files and require() always resolves
through ASSET.zip.

assets/scripts/PlayerCamera.js

@@ -0,0 +1,38 @@
+// Copyright (c) 2026 Dominic Masters
+//
+// This software is released under the MIT License.
+// https://opensource.org/licenses/MIT
+
+var CAMERA_OFFSET_ANGLE = 0.0;
+var CAMERA_OFFSET_RADIUS = 18.0;
+var CAMERA_OFFSET_HEIGHT = 10.0;
+
+class PlayerCamera {
+  constructor(target) {
+    this.target = target;
+
+    this.entity = new Entity();
+    this.position = this.entity.add(POSITION);
+    this.entity.add(CAMERA);
+
+    this.update();
+  }
+
+  update() {
+    var targetPosition = this.target.position.getLocalPosition();
+
+    var eyeX = targetPosition.x + Math.cos(CAMERA_OFFSET_ANGLE) *
+      CAMERA_OFFSET_RADIUS;
+    var eyeY = targetPosition.y + CAMERA_OFFSET_HEIGHT;
+    var eyeZ = targetPosition.z + Math.sin(CAMERA_OFFSET_ANGLE) *
+      CAMERA_OFFSET_RADIUS;
+
+    this.position.lookAt(
+      eyeX, eyeY, eyeZ,
+      targetPosition.x, targetPosition.y, targetPosition.z,
+      0.0, 1.0, 0.0
+    );
+  }
+}
+
+module.exports = PlayerCamera;