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/Player.js

@@ -3,23 +3,21 @@
 // This software is released under the MIT License.
 // https://opensource.org/licenses/MIT
 
-// Player: dynamic capsule body, moved relative to the camera by PLAYER's
-// own update callback (see entityplayer.c).
-class Player {
+class Player extends Entity {
   constructor() {
-    this.entity = new Entity();
+    super();
 
-    this.position = this.entity.add(POSITION);
+    this.position = this.add(POSITION);
     this.position.setLocalPosition(0.0, 2.0, 0.0);
 
-    this.physics = this.entity.add(PHYSICS);
+    this.physics = this.add(PHYSICS);
     this.physics.setShape(PHYSICS_SHAPE_CAPSULE, 0.5, 0.5);
 
-    this.renderable = this.entity.add(RENDERABLE);
+    this.renderable = this.add(RENDERABLE);
     this.renderable.setMesh(0, MESH_CAPSULE);
     this.renderable.setColor(0, 0, 255, 255);
 
-    this.entity.add(PLAYER);
+    this.add(PLAYER);
   }
 }