Separate test environment at /test.html (own Vite entry, own Phaser
instance). ZoomTestScene renders a 50×50 tile grid with crosshair
markers and a live HUD overlay showing zoom, scroll, viewport in px
and tiles, mouse world/screen/tile coords, and renderer info.
Zoom uses plain cam.setZoom() — no mouse tracking — to observe
Phaser's default center-anchor behavior.
