refactor: collapse toZod's enum leaf normalizer to a dummy union (#6533)

Unioning both sides with a private unique symbol forces TypeScript to rebuild each union, which is enough to make an enum reference and the union of exactly its members compare identical — the mapped-key rebuild and its Extract/Exclude split were doing the same job with more machinery. Differentially probed against the previous implementation across 35 cases (fixes, guarantees, numeric-enum nominality, classes with private members) on TypeScript 5.5, 6 and 7.0: observable behavior is identical.