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

tests/brep/CMakeLists.txt

@@ -31,3 +31,4 @@ add_vde_test(test_parallel_boolean)
 add_vde_test(test_feature_recognition)
 add_vde_test(test_v5_features)
 add_vde_test(test_v5_1)
+add_vde_test(test_advanced_blend)