Blender planet surface textures + atmosphere for close-ups

[ThomasAFink]

Summary

Upgrade Blender planet close-ups from shaded UV spheres to recognizable planet surfaces (starting with Earth), including clouds and a shared optional atmosphere limb haze.

Depends on

• First Blender planet close-up (shaded sphere) #12 First Blender planet close-up (shaded sphere + light/dark GIFs)

Scope

• Add Earth surface texture (and optional cloud/specular maps) into the close-up material pipeline under data/textures/bodies/<bodyId>/
• Shared appearance registry (body_appearance.py) so Moon / Jupiter / asteroids drop in later
• Optional cloud layer from clouds.png (mixed over surface color when present)
• Optional per-body atmosphere shell (fresnel limb haze): Earth on; asteroids stay off
• Keep CLI reproducible: render.py blender --body Earth --flyby (flag name may stay for now)
• Document texture source / license and where assets live in the repo
• Re-render light/dark gallery GIFs with textured Earth + clouds + atmosphere
• Leave room for other bodies without requiring all of them here

Acceptance

• Earth close-up shows continents / oceans (not a flat catalog-color sphere)
• Earth close-up shows a cloud layer from the shared pack (clouds.png)
• Earth close-up shows a thin atmosphere limb (shared BodyAtmosphere, not a one-off hack)
• Light/dark GIFs regenerate from CLI and update the README gallery
• Texture assets and attribution are documented
• Single-sun lighting still produces a clean terminator (no wedge artifact)
• Clouds / atmosphere are opt-in per body so airless packs (Moon, asteroids) stay clean

Related

Roadmap: Blender-based planet close-ups
PR: #40