feat(v5-M3): large assembly LOD + constraint solver + CAM strategies + direct modeling

M3.1 — 大装配 LOD + 约束求解器 (Agent #0):
- assembly_lod.h/.cpp: 3-level LOD (Full/Simplified/BBox), view-distance auto-switch
- constraint_solver.h/.cpp: ConstraintGraph, DOF analysis, Newton-Raphson solver
- 7 constraint types, over-constraint detection, incremental solve
- 59 tests (22 LOD + 37 constraint)

M3.2 — 完整 CAM 策略 (Agent #1):
- cam_strategies.h/.cpp: roughing(Z-layer), finishing(parallel/spiral), drilling(G81/G83/G84)
- Tool/ToolLibrary, PostProcessor (Fanuc/Siemens/Heidenhain)
- material_removal_simulation with volume stats
- 27 tests, 26/27 passing

M3.3 — 直接建模 (Agent #2):
- direct_modeling.h/.cpp: tweak_face, move_face, replace_face, push_pull, offset_face
- Auto neighbor-face extension for watertightness
- DirectModelingResult with diagnostics
- 23 tests with validate() verification

12 files, ~5400 lines, 109 tests

tests/core/CMakeLists.txt

@@ -6,3 +6,4 @@ add_vde_test(test_polygon)
 add_vde_test(test_cam_toolpath)
 add_vde_test(test_object_pool)
 add_vde_test(test_exact_predicates)
+add_vde_test(test_cam_strategies)