Add cathedral ruins scene, generator, render script, and design docs

[lienardale]

Summary

• Create a massive 236-shape scene (scenes/cathedral_ruins.rt) depicting a ruined medieval cathedral interior and render it at 2560x1440 as the README hero image
• Add cathedral preset to tools/gen_scene.c with 5 new shape writers (disk, torus, ellipsoid, triangle, paraboloid) and material-aware variants, enabling procedural regeneration with seed-controlled debris
• Create scripts/render_cathedral.sh with --preview/--full/--generate-only modes, xvfb headless rendering, and elapsed time reporting
• Create skills/scene-design.md documenting architectural scene design principles, the cathedral as a case study, material/lighting strategy, performance considerations, and the preview-iterate-render workflow
• Add render-cathedral Makefile target

Scene details

• 236 geometric shapes: 68 spheres, 49 boxes, 31 cylinders, 26 disks, 20 torus, 16 triangles, 13 squares, 10 ellipsoids, 1 plane, 1 paraboloid, 1 cone
• Materials: reflective puddles, translucent stained glass, specular stone, metallic cross
• 6 lights: warm sunbeams through roof gaps, cool sky fill, altar glow

Usage

# Generate + render at preview resolution
make render-cathedral ARGS="--preview"

# Generate + render at full resolution
make render-cathedral ARGS="--full"

# Just generate the .rt file
scripts/render_cathedral.sh --generate-only --seed 42

# Use gen_scene directly
./gen_scene --preset cathedral --width 2560 --height 1440 --output my_scene.rt

Test plan

• make gen_scene builds successfully
• ./gen_scene --list shows cathedral preset
• ./gen_scene --preset cathedral --output test.rt generates valid .rt file (~242 shapes)
• Generated scene parses without errors in miniRT
• scripts/render_cathedral.sh --preview renders BMP
• make render-cathedral ARGS="--full" produces saves/cathedral_ruins.bmp
• Verify README displays the new hero image on GitHub

https://claude.ai/code/session_011y4jf3XH1xLchcQXdo1NAy