chore(repo): drop darwin from toolchain flake

[ryanleecode]

Removes macOS/darwin from the Nix toolchain flake. No Mac build is kept anywhere.

Changes

• flake.nix: drop x86_64-darwin and aarch64-darwin from systems, leaving x86_64-linux and aarch64-linux.
• nix/dprint.nix: drop the two darwin release archives (platforms derives from the remaining Linux entries, so it narrows automatically).

Not touched

• flake.lock has no darwin-specific inputs — unchanged.
• pnpm-lock.yaml darwin entries are transitive optional-dep platform binaries (normal cross-platform lockfile data), not flake/CI macOS config — out of scope.
• Workflows already run only on ubuntu-latest hosted Linux runners.

Verified

• flake.nix and nix/dprint.nix parse clean (nix-instantiate --parse).
• All 5 workflow YAMLs parse.
• No remaining darwin/macos/apple reference in flake.nix or nix/*.nix.

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