build: add numpy and native library dependencies

Add numpy to requirements.txt and configure flake.nix with zlib and
stdenv.cc.cc.lib to support C extension packages. Sets LD_LIBRARY_PATH
for proper runtime linking.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

flake.nix

@@ -20,6 +20,14 @@
           packages = [
             python
             pkgs.python311Packages.virtualenv  # gives `virtualenv` tool
+            pkgs.zlib
+            pkgs.stdenv.cc.cc.lib
+          ];
+
+          # Required for numpy and other C extension packages
+          LD_LIBRARY_PATH = pkgs.lib.makeLibraryPath [
+            pkgs.zlib
+            pkgs.stdenv.cc.cc.lib
           ];
 
           shellHook = ''