improve validation for when uuid is not valid

[matt-beanland]

Combine allowed an artefact to be combined with no uuid (an empty string) which caused downstream problems. Improve the validator.

[matt-beanland]

#30 — improve validation for when uuid is not valid (closed in 0.1.5)

Validation now rejects empty/invalid uuids at op input time rather
than letting them propagate and cause silent corruption later in a
pipeline (the engines.livemd symptom that prompted this ticket).

Artefact.UUID.valid?/1 was added — checks UUIDv7 format
(8-4-4-4-12 hex, version digit 7, variant in {8,9,a,b}). Used
internally by the validation rule-set and exposed for callers.

Every public op (new/1, compose/3, combine/3, harmonise/4,
graft/3) validates its inputs and the produced artefact via the
new Artefact.validate!/1 (in 0.1.5) and via Validator.validate/1
through with clauses (in 0.2.0). Empty uuid raises
Artefact.Error.Invalid with a structured reason now, rather than
silently corrupting downstream graphs.