Add Nix devshell for Linux and macOS development (#1044)

- Restructure flake to expose `devShells.<system>.default` across
  x86_64-linux, aarch64-linux, x86_64-darwin, and aarch64-darwin via
  `nixpkgs.lib.genAttrs`. The existing `packages.x86_64-linux.default`
  build is preserved (still tied to the linux-x86_64 dawn/nod prebuilts).
- Linux devshell (`mkShell`): gcc + clang/lld, cmake, ninja, pkg-config,
  python3 + markupsafe, rustc/cargo, sccache, plus the system libs
  mirrored from the Ubuntu apt list in .github/workflows/build.yml
  (X11/Wayland, Vulkan, GL, ALSA/PulseAudio/PipeWire, GTK3, freetype,
  zstd, ...).
- macOS devshell (`mkShellNoCC`): cmake, ninja, python3 + markupsafe,
  rustc/cargo, sccache. No cc-wrapper so CMake picks up Apple Clang and
  the Xcode SDK directly, matching the build-apple CI job.
- Ignore `.direnv/` and `.envrc` so local direnv state stays out of git.

.gitignore

@@ -41,6 +41,10 @@ compile_commands.json
 # MacOS
 .DS_Store
 
+# direnv / nix
+.direnv/
+.envrc
+
 # ISOs
 *.iso