More procedural material recipes (metal, car paint, glass, leather...)

## Motivation Material fidelity is the single biggest lever on how "real" a reconstruction looks, but the documented recipe library is still small. This is one of the most contributor-friendly ways to improve every future model at once. ## Scope Add documented, parameterized **procedural material recipes** (independent PBR channels, no albedo aliased into roughness), each with a parameter table and a tiny demo: - Brushed / anisotropic metal - Automotive paint (clearcoat + metallic flake) - Subsurface wax / candle - Frosted / rough glass - Worn leather - Anodized aluminium ## Pointers - The material/lighting realism rubric in `references/` - `scripts/extract_reference_pbr.py` (reference-derived PBR evidence) ## Acceptance criteria - Each recipe lands as a documented block with parameters + a minimal before/after render. - Recipes use real, separable PBR channels and real lights. **Difficulty:** good first issue · pick a single material and open a focused PR. Ideal first contribution.