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.

src/CMakeLists.txt

@@ -106,6 +106,19 @@ target_link_libraries(vde_boolean
     PUBLIC  vde_mesh vde_spatial vde_compile_options
 )
 
+# ── sdf ────────────────────────────────────────────
+add_library(vde_sdf STATIC
+    sdf/sdf_tree.cpp
+    sdf/sdf_to_mesh.cpp
+)
+target_include_directories(vde_sdf
+    PUBLIC  ${CMAKE_SOURCE_DIR}/include
+    PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}
+)
+target_link_libraries(vde_sdf
+    PUBLIC  vde_core vde_compile_options
+)
+
 # ── collision ───────────────────────────────────────
 add_library(vde_collision STATIC
     collision/gjk.cpp
@@ -131,6 +144,7 @@ target_link_libraries(vde
     INTERFACE vde_spatial
     INTERFACE vde_boolean
     INTERFACE vde_collision
+    INTERFACE vde_sdf
     INTERFACE vde_brep
     INTERFACE vde_sketch
 )