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.

flake.nix

@@ -1,5 +1,5 @@
 {
-  description = "SolidCraft — native app development shell and package for Linux";
+  description = "SolidCraft — native app development shell and package for Linux and macOS";
 
   inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
 
@@ -9,10 +9,11 @@
       systems = [
         "x86_64-linux"
         "aarch64-linux"
+        "aarch64-darwin"
       ];
       forAllSystems = f: nixpkgs.lib.genAttrs systems (system: f nixpkgs.legacyPackages.${system});
 
-      # Why this shell exists. winit opens the window by dlopen'ing the Wayland client libraries,
+      # Linux only. Why this shell exists. winit opens the window by dlopen'ing the Wayland client libraries,
       # wgpu renders through the EGL loader (libEGL.so.1), and rfd's file dialogs reach
       # xdg-desktop-portal through libdbus-1; a plain `cargo run` under NixOS finds none of them,
       # because there is no /usr/lib and nothing puts them on LD_LIBRARY_PATH:
@@ -29,34 +30,44 @@
       # `xvfb-run` on a machine with no display at all.
       runtimeLibs =
         pkgs:
-        (with pkgs; [
-          wayland
-          libxkbcommon
-          libglvnd
-          vulkan-loader
-          dbus
-        ])
-        ++ (with pkgs; [
-          # xorg.libX11 and friends are deprecated aliases in current nixpkgs.
-          libx11
-          libxcursor
-          libxi
-          libxrandr
-          libxrender
-          libxext
-          libxcb
-        ]);
-      driverLibDir = "/run/opengl-driver/lib";
-      driverShareDir = "/run/opengl-driver/share";
-      libraryPath = pkgs: "${pkgs.lib.makeLibraryPath (runtimeLibs pkgs)}:${driverLibDir}";
+        pkgs.lib.optionals pkgs.stdenv.isLinux (
+          (with pkgs; [
+            wayland
+            libxkbcommon
+            libglvnd
+            vulkan-loader
+            dbus
+          ])
+          ++ (with pkgs; [
+            # xorg.libX11 and friends are deprecated aliases in current nixpkgs.
+            libx11
+            libxcursor
+            libxi
+            libxrandr
+            libxrender
+            libxext
+            libxcb
+          ])
+        );
+      # macOS needs none of the above: winit talks to AppKit, wgpu renders through Metal, and the
+      # OS ships its GPU drivers, so the darwin package and shells skip the wrapper and paths.
+      driverLibDir = pkgs: if pkgs.stdenv.isLinux then "/run/opengl-driver/lib" else "";
+      driverShareDir = pkgs: if pkgs.stdenv.isLinux then "/run/opengl-driver/share" else "";
+      libraryPath =
+        pkgs:
+        if pkgs.stdenv.isLinux then "${pkgs.lib.makeLibraryPath (runtimeLibs pkgs)}:${driverLibDir pkgs}" else "";
 
-      shellHook = ''
-        if [ ! -d ${driverLibDir} ]; then
-          echo "SolidCraft: ${driverLibDir} is missing — on NixOS enable hardware.graphics; on other distributions use a matching nixGL driver wrapper." >&2
-        fi
+      shellHook = pkgs: ''
+        ${pkgs.lib.optionalString pkgs.stdenv.isLinux ''
+          if [ ! -d ${driverLibDir pkgs} ]; then
+            echo "SolidCraft: ${driverLibDir pkgs} is missing — on NixOS enable hardware.graphics; on other distributions use a matching nixGL driver wrapper." >&2
+          fi
+        ''}
         command -v cc >/dev/null || echo "solvecraft: no C linker on PATH, cargo cannot link — use nix develop .#rust" >&2
         echo "solvecraft: cargo run --release -p solvecraft -- --sample"
-        export XDG_DATA_DIRS="${driverShareDir}:''${XDG_DATA_DIRS:-/usr/local/share:/usr/share}"
+        ${pkgs.lib.optionalString pkgs.stdenv.isLinux ''
+          export XDG_DATA_DIRS="${driverShareDir pkgs}:''${XDG_DATA_DIRS:-/usr/local/share:/usr/share}"
+        ''}
       '';
     in
     {
@@ -68,7 +79,7 @@
         default = pkgs.mkShellNoCC {
           name = "solidcraft";
           LD_LIBRARY_PATH = libraryPath pkgs;
-          inherit shellHook;
+          shellHook = shellHook pkgs;
         };
 
         # For a machine with no Rust at all: `nix develop .#rust`. Its own target directory, so
@@ -85,7 +96,7 @@
           # Under target/, which .gitignore already covers.
           CARGO_TARGET_DIR = "target/nix";
           shellHook = ''
-            ${shellHook}
+            ${shellHook pkgs}
             echo "solvecraft: $(rustc --version), building into target/nix"
           '';
         };
@@ -123,25 +134,27 @@
           buildInputs = runtimeLibs pkgs;
 
           postInstall = ''
-            install -Dm644 packaging/linux/ai.storyteller.solvecraft.desktop \
-              $out/share/applications/ai.storyteller.solvecraft.desktop
-            install -Dm644 packaging/linux/ai.storyteller.solvecraft.mime.xml \
-              $out/share/mime/packages/ai.storyteller.solvecraft.xml
-            for size in 16 24 32 48 64 128 256 512; do
-              icon=$out/share/icons/hicolor/''${size}x''${size}/apps/ai.storyteller.solvecraft.png
-              mkdir -p "$(dirname "$icon")"
-              $out/bin/solvecraft-cli icon --size "$size" --out "$icon" >/dev/null
-            done
             ln -s solvecraft $out/bin/solidcraft
+            ${pkgs.lib.optionalString pkgs.stdenv.isLinux ''
+              install -Dm644 packaging/linux/ai.storyteller.solvecraft.desktop \
+                $out/share/applications/ai.storyteller.solvecraft.desktop
+              install -Dm644 packaging/linux/ai.storyteller.solvecraft.mime.xml \
+                $out/share/mime/packages/ai.storyteller.solvecraft.xml
+              for size in 16 24 32 48 64 128 256 512; do
+                icon=$out/share/icons/hicolor/''${size}x''${size}/apps/ai.storyteller.solvecraft.png
+                mkdir -p "$(dirname "$icon")"
+                $out/bin/solvecraft-cli icon --size "$size" --out "$icon" >/dev/null
+              done
+            ''}
           '';
 
-          # The app reaches the window through dlopen'd libraries, so the packaged binary needs the
-          # same LD_LIBRARY_PATH the shell exports.
-          postFixup = ''
+          # Linux only: the app reaches the window through dlopen'd libraries, so the packaged
+          # binary needs the same LD_LIBRARY_PATH the shell exports. macOS needs no wrapper.
+          postFixup = pkgs.lib.optionalString pkgs.stdenv.isLinux ''
             wrapProgram $out/bin/solvecraft \
               --prefix LD_LIBRARY_PATH : "${libraryPath pkgs}" \
               --set-default XDG_DATA_DIRS /usr/local/share:/usr/share \
-              --prefix XDG_DATA_DIRS : "$out/share:${driverShareDir}"
+              --prefix XDG_DATA_DIRS : "$out/share:${driverShareDir pkgs}"
           '';
 
           meta = {