refactor(maplibre-effects): render atmosphere behind the globe (#440)

* refactor(maplibre-effects): render atmosphere behind the globe

Replace the on-top silhouette punch-out with background canvases layered
behind the MapLibre canvas, so the space, starfield, comet, and halo
effects show through the globe's transparent space instead of masking it.
This drops the per-frame ellipse-fitting round-trips and the zoom fade in
favor of a simpler tiled parallax starfield and great-circle halo.

* chore: update .gitignore to exclude 'private/' directory

Author: giswqs