stable compact api

[matt-beanland]

use defdelegate to create a stable, compact API

https://medium.com/@nature.exs/elixirs-defdelegate-an-underrated-tool-for-stable-apis-90a40618febf

intent is that although we may break existing consumers we don't ever have to again.
our API users shouldn't see or care about our internal module structure

[matt-beanland]

#23 — stable compact API (closed in 0.2.0 — breaking)

Artefact is now a thin macro facade:

• Operations live in Artefact.Op (internal).
• Validation lives in Artefact.Validator (internal, surfaced via
  defdelegate).
• Error structs live in Artefact.Error.*.

Future internal refactors won't churn the consumer-visible surface.
Callers always write Artefact.combine(...) regardless of where the
implementation lives.

The op surface uses macros (not defdelegate) because the macros
capture __CALLER__.module for free provenance. The intent of #23 —
"users don't see the internal module structure" — is met either way.