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.
Every bright star in sky_nebula.png reused the exact same diffraction-
spike stamp, just relocated. Commit the generator (recovered from an
ephemeral scratchpad) to tools/textures/gen_nebula_sky.py, randomize
each hero star's rotation, arm count, spike length, and brightness,
and regenerate the texture.