feat(v5-M2): G2/G3 continuity + surface analysis + extension + N-side fill + advanced blend

M2.1 — G2/G3 连续性分析 (Agent #0):
- surface_continuity.h/.cpp: G0/G1/G2/G3 curve/surface detection
- Weingarten equation for curvature, Frénet frame, zebra stripe
- surface_analysis.h/.cpp: curvature_map, deviation_analysis, curvature_comb
- 24 tests (12 continuity + 12 analysis)

M2.2 — 曲面延伸 + N边填充 (Agent #1):
- surface_extension.h/.cpp: extend_surface(G1/G2), n_sided_fill, blend_surfaces
- Coons patch generalization for N-sided holes
- 16/16 tests passed in Docker container

M2.3 — 高级过渡曲面 (Agent #2):
- advanced_blend.h/.cpp: real implementations replacing stubs
- variable_radius_blend, multi_face_blend, rolling_ball_blend, face_face_blend
- Ball-rolling envelope + corner sphere filling
- 15+ tests with validate() verification

src/CMakeLists.txt

@@ -52,6 +52,9 @@ add_library(vde_curves STATIC
     curves/parallel_intersection.cpp
     curves/surface_fairing.cpp
     curves/exact_offset.cpp
+    curves/surface_extension.cpp
+    curves/surface_continuity.cpp
+    curves/surface_analysis.cpp
 )
 target_include_directories(vde_curves
     PUBLIC  ${CMAKE_SOURCE_DIR}/include