Is the ID thats part of an identity guaranteed to be unique and static per user and per Kratos instance? e.g.

// Identity example
{
  id: "6e9d3d30-f93e-4630-901f-c2096953723d",
  traits: {
    username: "some-username",
  },
}

I want to link Kratos identities to subjects in Keto, and any data like e-mail or username are not static 🤔