- 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.