Make nix develop work on MacOS (apple silicon)

[jscheidtmann]

Enables nix develop for local development on macOS Apple Silicon.

Summary

• flake.nix — adds devShells.aarch64-darwin.default pointing at the new darwin Python environment
• nixos/pkgs/uv-python-darwin.nix — new file: uv2nix workspace expression for macOS; includes only the overrides needed on Darwin (timezonefinder, sh, and cedar-solve each lack a setuptools build-system declaration)
• python/pyproject.toml — adds "sys_platform == 'darwin'" to [tool.uv] environments; gates Linux-only hardware packages (spidev, rpi-gpio, picamera2, pidng, dbus-python, pygobject, python-libinput, python-prctl, python-pam, adafruit-*, videodev2) behind sys_platform == 'linux'
• python/uv.lock — regenerated to include macOS (arm64/universal2) wheels and update dependency markers to sys_platform == 'darwin' or sys_platform == 'linux' for cross-platform packages

Test plan

• nix develop --command echo OK exits 0 on aarch64-darwin
• python -c "import flask, numpy, skyfield, grpc, tetra3, scipy" succeeds inside the shell
• Linux nix develop (aarch64-linux) still works — platform-gated markers unchanged