Implements scroll correction after cam.setZoom() so the world point
under the mouse stays fixed. Formula accounts for Phaser's
center-based zoom: scrollX += (mouseX - cw/2) * (1/zBefore - 1/zAfter).
Tab switches between the two test scenes in both directions.
Also fixes centerWorld formula in ZoomTestScene overlay and logs.
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.
