feat(examples): add 3D print pipeline, B-Rep fab, and SDF optimization demo

- 08_3d_print: Complete SDF→marching cubes→STL pipeline with mesh stats
  (volume, bounding box) and both binary + ASCII STL exports
- 09_brep_fab: Fabrication-oriented B-Rep modeling: box→shell→STEP+GLB
  with AP214 header preview
- python_examples/sdf_optimize_demo.py: SDF parameter optimization
  from point clouds with analytic gradient descent
- Update examples/CMakeLists.txt to include new subdirectories
- Add __pycache__/ to .gitignore

.gitignore

@@ -9,3 +9,4 @@ compile_commands.json
 *.gcno
 *.gcov
 coverage_report/
+__pycache__/