Summary

Implements the 3D Viewport (Feature 2 of 10) with full main window shell. The application is now end-to-end usable: open a mesh file and interact with it in 3D.

What's included

• VtkMeshAdapter — vectorized MeshData → vtkPolyData conversion using numpy bridge (~800x faster than naive loops)
• SceneManager (B-lite pattern) — owns mesh actor, wireframe overlay actor, headlight + ambient fill lighting, camera fit-to-view, flat/smooth shading toggle
• ViewportWidget — QVTKRenderWindowInteractor subclass with empty/error state overlays
• MainWindow — full rewrite with File/View/Help menus, vertical left toolbar (icon+text per accessibility), status bar, state management (empty/loading/success/error)
• File loading integration — file dialog (Ctrl+O), drag-drop, CLI argument (meshscope model.stl)
• Keyboard shortcuts — W (wireframe overlay), S (flat/smooth shading), F (fit to view), Ctrl+O (open), Ctrl+Q (exit)
• Accessibility — all toolbar buttons have text labels, all actions have tooltips for screen readers, keyboard navigation for all functions
• 60 new tests (121 total) — unit tests for adapter + scene manager, UI tests for viewport + main window + file loading integration

Architecture decision

B-lite SceneManager establishes the seam for Features 5, 9, 10 (print bed, measurements, clipping plane) to add actors without refactoring ViewportWidget. Minimal API now, extensible later.

Test plan

• 121 tests pass (QT_QPA_PLATFORM=offscreen pytest tests/ -v)
• All pre-commit hooks pass (ruff, semgrep, gitleaks)
• STL/OBJ/PLY/3MF all load through full pipeline (integration tests)
• Error states tested (corrupt files, missing files, error-after-success)
• Manual visual verification (mesh renders, rotates, wireframe overlays)
• Verify CI passes

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