Add macOS support

[etjones]

Summary

Adds macOS as a supported platform. The workbench code was already portable; this PR contributes a cross-platform per-user installer, platform-aware filesystem paths, docs, and a macOS CI leg.

• New scan2sketch/platform_paths.py — resolves the venv interpreter layout (bin/python vs Scripts\python.exe), the per-user cache root (~/Library/Caches / %LOCALAPPDATA% / XDG), and the FreeCAD user-data base. All previously Windows-hardcoded path sites in external_backend, calibration_runtime, diagnostics, calibration_profile, and the calibration dialogs now go through it. Windows behavior is unchanged.
• New scripts/install_user_addon.py / uninstall_user_addon.py — stdlib-only ports of the PowerShell installers for macOS/Linux: same copy/dev-link modes, safety stops, SHA-256-verified model downloads, and isolated venvs built from FreeCAD's bundled Python. The installer asks freecadcmd for App.getUserAppDataDir() so it keeps targeting the right versioned directory (v1-1, v1-2, ...) across FreeCAD upgrades, and finds the bundle Python inside /Applications/FreeCAD*.app automatically. On Windows these scripts defer to the existing .ps1 files.
• Declared the missing shapely test dependency in requirements-opencv-spike.txt. scan2sketch/core/offset.py relies on the Shapely that FreeCAD ships at runtime, but the FreeCAD-independent test environment never declared it, so test_offset.py's robust-offset test fails in a fresh CI-style environment on every platform (same situation SciPy was in before 7367ce1). Pinned to 2.1.2 to match FreeCAD 1.1.1's bundled version.
• CI — core-tests.yml now runs a windows-latest + macos-latest matrix.
• Docs — new INSTALL_MACOS.md (linked from README/INSTALL.md), including honest platform floors: macOS 13+ on Apple silicon, macOS 14+ on Intel (the opencv wheel floor), and OpenCV-mode-only on Intel Macs since PyTorch publishes no Intel macOS wheels for the pinned torch — the installer fails fast with that explanation.

Testing

• Full 198-test suite passes on macOS (Apple silicon) in a venv built by the new installer from FreeCAD 1.1.3's bundled Python 3.11, and on both CI legs.
• End-to-end in FreeCAD 1.1.3 on macOS: install (dev-link and copy modes), workbench registration, first-run wizard, grid calibration of a synthetic 600 dpi scan (quality "high", 0.017 mm residual, profile saved to App.getUserAppDataDir()), scan analysis through the isolated OpenCV helper, and sketch creation (5 geometries, ±0.082 mm reported uncertainty). Diagnostic export and uninstall verified.
• Not yet tested on macOS: --full-vision (SAM) runtime and real-scanner input.

Notes for maintainers

• Heads-up beyond this PR: torch==2.5.1 has no cp313 wheels on any platform, so if a future FreeCAD bundles Python 3.13 the full-vision install breaks on Windows too; the pin will need a bump at that point.
• A German translation of INSTALL_MACOS.md is not included; happy to follow up if wanted.

🤖 Generated with Claude Code

[etjones]

(BTW, I pointed Claude at this repo when I saw there was a need for macOS as well, and it set up the PR above. But I'm a real guy, looking to support this as needed. Thanks for putting together the workbench!)

[Jayktopus]

Thank you — the macOS support has been merged after all 198 tests passed on both Windows and macOS. I added a visible follow-up note to the English and German documentation clarifying that the OpenCV workflow has been tested on Apple silicon, while the optional SAM / Light HQ-SAM full-vision path remains experimental on macOS until it receives equivalent end-to-end testing.