Extend gen_nebula_sky.py with two more dust-lane layers at different
scales and subtle hue variation within the bright core. Add
gen_planet_surface.py (no prior generator existed) producing latitude
bands, storm vortices, and a lit/unlit terminator baked from the
planet mesh's actual UV convention against arena_02's directional
light, verified in-engine via screenshots.
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
