DEV: add cross-compiled nix devshells for multiple architectures

- Define `arch_pkgs` mapping for native, aarch64, s390x, ppc64le, and armhf
- Allow selection via `NPSR_ARCH` env var (defaults to native)
- Configure per-arch virtualenv under `.direnv/py/<arch>`
- Bootstrap Python venv with `spin==0.13`

Author: seiko2plus

.gitignore

@@ -118,3 +118,6 @@ Thumbs.db
 # pytest generated files #
 ##########################
 /.pytest_cache
+
+# NIXOS generated files #
+.direnv/

default.nix

@@ -0,0 +1,31 @@
+let
+  pkgs = import <nixpkgs> { };
+  arch_pkgs = {
+    native = pkgs;
+    aarch64 = pkgs.pkgsCross.aarch64-multiplatform;
+    s390x = pkgs.pkgsCross.s390x;
+    ppc64le = pkgs.pkgsCross.powernv;
+    armhf = pkgs.pkgsCross.armv7l-hf-multiplatform;
+  };
+  arch = if builtins.getEnv "NPSR_ARCH" == "" then "native" else builtins.getEnv "NPSR_ARCH";
+  venv_path = ".direnv/py/" + arch;
+  pkgsCross = arch_pkgs.${arch};
+in
+pkgsCross.mkShell {
+  name = "Numpy SIMD Routines ${arch}";
+  packages = with pkgsCross; [
+    sollya
+    python312
+    python312Packages.virtualenv
+  ];
+  shellHook = ''
+    if test ! -d "${venv_path}"; then
+      python -m venv ${venv_path}
+      source ${venv_path}/bin/activate
+      python -m pip install --upgrade pip
+      python -m pip install "spin==0.13" 
+    else
+      source ${venv_path}/bin/activate
+    fi
+  '';
+}