build: macOS support for the Nix flake

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.

docs/nix.md

@@ -1,6 +1,6 @@
 # SolidCraft with Nix
 
-The flake supports `x86_64-linux` and `aarch64-linux`. Enable Nix's `nix-command`
+The flake supports `x86_64-linux`, `aarch64-linux` and `aarch64-darwin`. Enable Nix's `nix-command`
 and `flakes` experimental features. The committed `flake.lock` pins nixpkgs and
 its Rust toolchain; package evaluation checks that compiler against the Rust
 floor in `Cargo.toml`.
@@ -35,8 +35,9 @@ nix profile install github:bherbruck/solidcraft#solidcraft
 solidcraft --sample
 ```
 
-This installs the SolidCraft desktop entry, MIME definition and generated icon
-sizes alongside the commands. NixOS/Home Manager can instead put the flake's
+On Linux, this installs the SolidCraft desktop entry, MIME definition and generated icon
+sizes alongside the commands. On macOS the package contains the binaries only; for a bundled
+`SolveCraft.app`, use the release `.dmg`. NixOS/Home Manager can instead put the flake's
 `packages.${system}.solidcraft` in `environment.systemPackages`/`home.packages`.
 If a non-NixOS desktop does not discover profile-installed launchers, ensure that
 its session includes `~/.nix-profile/share` in `XDG_DATA_DIRS`, then log out and
@@ -64,6 +65,10 @@ discoverable. Enable `hardware.graphics.enable = true` on NixOS and keep the
 system's driver stack consistent with the compositor. File pickers also need a
 working session D-Bus and the desktop's `xdg-desktop-portal` service.
 
+On macOS (`aarch64-darwin`), none of this applies: winit talks to AppKit, wgpu renders through
+Metal, and the GPU drivers ship with the operating system, so the darwin package carries no
+library wrapper and no extra search paths.
+
 On another distribution, graphics drivers outside the Nix store may need a
 matching [nixGL wrapper](https://github.com/nix-community/nixGL). Follow its
 GPU-specific instructions; a wrapper for a different driver is not a substitute.