Move the nebula sky, planet surface and particle glow textures to VRAM
compression with mipmaps, so they stop being uploaded uncompressed.
The sky panorama is forced to high quality (BC7). It is a 5 MB image of
smooth nebula gradients seen through the arena walls, and BC1/BC3's 5:6:5
endpoint interpolation bands visibly across exactly that kind of content.
Introduce arena_02 (Nebula) and arena_03 (Asteroid Field) alongside
arena_01, listed in a new ArenaRegistry (scripts/arena_registry.gd) as
the single source of truth for available arenas. Free Play lets the
player pick an arena from the main menu; Match/Spectate each pick one
at random per session; Training keeps its own fixed arena_01.
Nebula gets an RL-style visual treatment: a near-invisible glass
boundary material shared by all arenas, a baked equirect nebula sky
texture, a drifting-dust particle system, and a Blender-modeled
station/debris/planet decoration set. GameMode gains a
_get_arena_scene_path() hook so modes can instantiate their arena in
code instead of hardcoding it in the scene.
Josh Creek
