UUID validator

Checks that a UUID has the right shape and reports its version and variant, both of which are encoded in the value itself rather than stored alongside it.

The version lives in the first digit of the third group and the variant in the first digit of the fourth, which is why a UUID carries its own metadata. Version 4 is random and by far the most common; version 1 is time-and-MAC based and leaks both the creation time and the network card of the machine that made it, which is a genuine privacy consideration if those UUIDs are exposed publicly. Version 7, standardised in RFC 9562, is the new time-ordered one and is worth preferring over v4 for database keys for the same reason a ULID is. A validator can only check structure — it cannot tell you whether a well-formed UUID was ever actually issued by your system.