Give overlays namespace argument

[jameslounds]

Closes #121

[anntnzrb]

Adopted

This fix has been implemented.

The namespace argument is now properly passed to overlays in create-overlays-builder, making it consistent with create-overlays.

Change

  create-overlay = overlay:
    import overlay (
      user-inputs
      // {
        inherit channels;
+       inherit (snowfall-config) namespace;
        inputs = user-inputs;
        lib = snowfall-lib.internal.system-lib;
      }
    );

Verification

✅ nix flake check passes
✅ Behavior now consistent across overlay functions
✅ Backwards compatible

Commit: anntnzrb/snowfall-lib@b85b94d