Replace Earth/Moon cinematic dots with Blender body graphics

[ThomasAFink]

Summary

In the Sol→α Cen matplotlib cinematic, replace the Earth and Moon scatter dots with graphics from the Blender close-up renders — drawn during FuncAnimation (not GIF concat). Optional CLI flag keeps the classic dotted look.

Depends on

• Blender planet surface textures + atmosphere for close-ups #36 Textured Earth close-ups
• Blender Moon close-up (texture pack) #41 Blender Moon close-up

Scope

• Load Earth/Moon Blender flyby frames from output/animate/blender/{planets,moons}/…
• While rendering the cinematic, billboard those graphics at Earth/Moon 3D positions (camera-facing), advancing frames as the animation plays
• Fall back to dots when zoomed out too far for a readable sprite, or if assets are missing
• CLI flag: render.py animate --system sol_centauri --blender-bodies (default off = current look)
• Earth + Moon only in this issue; other bodies later
• Do not post-process / prepend GIFs after matplotlib finishes

Acceptance

• Without --blender-bodies, cinematic matches today’s dotted Earth/Moon
• With --blender-bodies, Earth and Moon show Blender surface graphics in-scene as frames render
• Theme (light/dark) picks the matching flyby assets
• Missing Blender assets degrade gracefully to dots
• Documented in README / CLI help

Out of scope

• Full Blender rewrite of the Sol→α Cen camera path
• Billboards for other planets/moons (follow-up)

Related

Builds on #10, #36, #41.