Implementation home for Artefact operations.
Public ops are surfaced through Artefact itself via macros (which
capture __CALLER__.module for provenance). This module is the
function-form home — it takes caller as an explicit argument.
Every function returns {:ok, %Artefact{}} or {:error, _}. Errors
are Artefact.Error.Invalid (validation rule violations) or
Artefact.Error.Operation (op-specific outcomes — see the latter's
moduledoc for the per-op tag list).