chore: drop darwin from flake

[ryanleecode]

Removes macOS/darwin from the Nix toolchain so the flake targets Linux only.

• flake.nix: systems list drops x86_64-darwin / aarch64-darwin.
• nix/attw.nix: meta.platforms drops the two darwin entries.
• nix/dprint.nix: removes the x86_64-darwin / aarch64-darwin release entries (sha256 pins). platforms derives from the release set and isLinux guards already handle the Linux-only paths, so no further change is needed.

Workflows already run only on GitHub-hosted Linux runners (ubuntu/blacksmith); nothing to change there. flake.lock unchanged (no inputs touched).

Verified locally: nix flake show --all-systems evaluates cleanly to aarch64-linux + x86_64-linux only; all workflow YAML parses.

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