Provide libstdc++ to manylinux wheels in the Nix dev shell

greenlet (pulled in by pytest-playwright) ships a manylinux wheel whose
C extension links against libstdc++.so.6, which the nixpkgs Python
cannot resolve, breaking pytest at plugin-load time. Expose it via an
LD_LIBRARY_PATH scoped to the dev shell.

https://claude.ai/code/session_01BKurBhE3Qj25p7Bfsg7EeK

shell.nix

@@ -11,6 +11,12 @@ pkgs.mkShell {
     pnpm
   ];
 
+  # manylinux wheels with native extensions (greenlet, pulled in by
+  # pytest-playwright) link against libstdc++.so.6, which the nixpkgs
+  # Python cannot find on its default search path. Scoped to this dev
+  # shell only — a global LD_LIBRARY_PATH would leak into other programs.
+  LD_LIBRARY_PATH = pkgs.lib.makeLibraryPath [ pkgs.stdenv.cc.cc.lib ];
+
   shellHook = ''
     uv venv --clear
     . .venv/bin/activate