feat(sdf): S11-B — SDF operations + domain deformations
Add header-only SDF combinator operations and domain deformation
functions based on Inigo Quilez standard formulations:
Boolean operations:
- op_union (min), op_intersection (max), op_difference (max(-d1,d2))
- Smooth variants: op_smooth_union/intersection/difference with blend radius k
with graceful fallback to sharp when k <= 0
Shape modifiers:
- op_round (d - r), op_onion (abs(d) - t/2)
Domain deformations (coordinate remapping):
- op_repeat (infinite grid repetition)
- op_mirror_x/y/z (with optional offset)
- op_rotate (Y-axis), op_translate, op_scale
- op_twist (Y-axis), op_bend, op_cheap_bend
- op_elongate, op_displace
All functions are inline in include/vde/sdf/sdf_operations.h.
Comprehensive unit tests in tests/sdf/test_sdf_operations.cpp covering
sharp/smooth booleans, modifiers, domain ops, and composition tests.
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