Add aarch64-darwin to the flake systems so packages, apps and both
devShells are exposed on Apple Silicon Macs. The Wayland/X11/EGL
runtime libraries, the /run/opengl-driver search paths, the freedesktop
packaging and the wrapProgram post-fixup are Linux-only and stay that
way: on macOS winit talks to AppKit, wgpu renders through Metal, and
the GPU drivers ship with the OS, so the darwin package needs no
wrapper. x86_64-darwin is deliberately not added: current nixpkgs
unstable no longer evaluates rustc for that platform (26.11
deprecation), so the package would fail on toolchain evaluation.
docs/nix.md: platform list updated, macOS paragraph in the graphics
section, desktop-entry paragraph qualified as Linux-only.
Levon Kirakosyan