ci(nix): add startup smoke test

[xz-dev]

Summary

• add a nix smoke GitHub Actions workflow using cachix/install-nix-action@v31
• build .#neomacs, assert both canonical and fingerprinted pdump files are present in the Nix result, then run a batch startup smoke test
• install the final neomacs.pdump image into the Nix package output next to the wrapped binary so the Nix-built executable can start

Closes #133

Validation

• git diff --check
• python - <<'PY' ... yaml.safe_load('.github/workflows/nix-smoke.yml') ... PY
• gh act push -W .github/workflows/nix-smoke.yml -j nix-smoke -n
• Tried a real local gh act run with --privileged; Nix installed and started the build, but the local container build was stopped after several minutes because it was rebuilding/downloading the full dependency graph. The dry-run validates the workflow wiring; the PR workflow should exercise the full nix build smoke path on GitHub-hosted runners.

[gemini-code-assist]

Code Review

This pull request updates the Nix flake to verify and install the final neomacs.pdump image, extracting its fingerprint by executing the compiled binary. The review feedback correctly points out that executing the target binary directly during the build phase breaks cross-compilation, and suggests using an emulator prefix to resolve this issue.

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[gemini-code-assist]

Executing the target binary directly ($out/bin/neomacs) during the build phase breaks cross-compilation, as the build host cannot execute binaries compiled for a different target architecture (e.g., building aarch64-linux on x86_64-linux).

To support cross-compilation, we should prefix the execution with the appropriate emulator for the target platform using ${pkgs.stdenv.hostPlatform.emulator pkgs}.

              fingerprint="$(${pkgs.stdenv.hostPlatform.emulator pkgs} $out/bin/neomacs --fingerprint | tr -d '[:space:]')"

[xz-dev]

Thanks, fixed in the latest push. I used ${pkgs.stdenv.hostPlatform.emulator pkgs.buildPackages} when invoking the just-built $out/bin/neomacs --fingerprint, matching the nixpkgs cross-compilation pattern for executing host-platform binaries from a build.

[eval-exec]

Thank you!