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.
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