feat(assets): add Sol system handcrafted terrain pipeline

Custom pipeline for GJ-0 (Sol) that imports real NASA/USGS
planetary data instead of procedural generation. Produces the
same output format (heightmap.png, globe.png, markers.json).

Real data bodies:
- Earth: ETOPO2022 elevation + WorldClim climate + 14 rivers
- Mars: MOLA DEM + ferric biome classes + terraformed water
- Luna: LOLA DEM + lunar biome palette

Procedural fallback for Mercury, Venus, Phobos, Deimos.
Synthetic elevation from albedo for Io, Europa, Ganymede,
Callisto, Titan, Enceladus. Gas giants use existing renderer.

New biome classes 34-36 (ferric_dust/highland/lowland) for
Mars iron oxide surface. Earth features: 50 cities (smart
scatter by continent), 15 named rivers, oceans, mountains.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -39,6 +39,8 @@ spikes/**/*.npz
 
 # Planet generator intermediates
 tooling/planet-gen/__pycache__/
+tooling/planet-gen/sol_data/.cache/
+tooling/planet-gen/sol_data/__pycache__/
 *.tmp.npz
 
 # Generated terrain grids (large, regenerated from pipeline)