Add a home.uid option similar to home.username. When set, the
activation script verifies the current UID matches the expected
value using the new checkUid function.
When using the NixOS or nix-darwin modules, home.uid is
auto-discovered from users.users.<name>.uid when that value
is set.
This is useful for constructing paths that depend on the user's
UID, such as /run/user/<uid> paths for gpg-agent sockets or
other user-specific runtime directories.
