feat(shaders): Add optimized 4-sample SDF normal estimation

Adds get_normal_fast() variant using tetrahedral gradient approximation.

Performance improvement:
- 4 SDF evaluations (vs 6 in get_normal_basic)
- ~33% fewer distance field samples
- Tetrahedron pattern: k.xyy, k.yyx, k.yxy, k.xxx where k=(1,-1)

Trade-off:
- Slightly less accurate than central differences method
- Good for real-time rendering where performance matters
- Same sphere optimization (analytical normal for obj_type == 1.0)

Parameters:
- epsilon: 0.0001 (vs 0.001 in basic method)
- Same interface: takes position and object params

Use case: Fast lighting calculations, performance-critical shaders

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