feat(export): Add STL export with parameter dialog (v0.6.0)

- StlExportService: binary/ASCII STL writer (degenerate triangle
  filtering, per-triangle normals) + B-rep re-tessellation
- StlExportDialog + StlExportViewModel: scope (merged/per-file),
  format, unit scale (mm/inch/custom), mesh quality (current/fine),
  live triangle count + size estimate; confirm before export
- Background export with UI-thread mesh snapshot (GeometryModel3D is
  DispatcherObject; only frozen MeshGeometry3D crosses threads)
- Fine quality = 0.15x import precision: CADability GetTriangulation
  returns cached mesh for coarser requests (no coarse option) and
  triangle count is non-monotonic in precision (0.4x < 1x)
- SmokeTest --stl-export-test: binary round-trip incl. inch scaling,
  ASCII structure, degenerate filtering, real-file fine re-tessellation
- Bump version to 0.6.0; update ARCHITECTURE/CLAUDE docs (Phase 15)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tools/SmokeTest/Program.cs

@@ -38,6 +38,9 @@ if (args[0] == "--bbox")
 if (args[0] == "--export-test")
     return SmokeTest.ExportTest.Run(args[1], args[2]) == 0 ? 0 : 2;
 
+if (args[0] == "--stl-export-test")
+    return SmokeTest.StlExportTest.Run(args.Length > 1 ? args[1] : null) == 0 ? 0 : 2;
+
 var service = new StepImportService { Progress = s => Console.WriteLine($"  [階段] {s}") };
 int failed = 0;