Tags and identity

Every instance carries a non-enumerable _tag, for pattern matching with unthrown's P.tag(...):

match(member)
  .with(P.tag("User"), (u) => u.email)
  .with(P.tag("ServiceAccount"), (s) => s.label)
  .exhaustive();

It never reaches the wire — absent from every schema, from toJSON(), JSON.stringify, Object.keys and spread. That has a direct consequence: a union that must survive a JSON round trip cannot discriminate on _tag, because it is not there after serialisation. Declare the discriminant as an ordinary domain field; Entity.union takes that field.

The two are not redundant. A brand is per-field and type-only; the tag is per-entity and runtime-present, which is what makes it matchable. entityName is the same string read from the class rather than an instance — the only path for code holding the class and no instance.

Model an aggregate declares such a union against a real payload.