Optional behaviour for external signing adapters.
Core Rendro stops at prepared artifact output. Optional adapters can inspect that prepared artifact, derive adapter-local handoff data, and return updated bytes plus namespaced metadata without widening the shared core manifest.
Summary
Callbacks
@callback augment(Rendro.Artifact.t(), map()) :: {:ok, binary(), map()} | {:error, term()}
@callback prepare(Rendro.Artifact.t(), map()) :: {:ok, binary(), map()} | {:error, term()}
@callback sign(Rendro.Artifact.t(), map()) :: {:ok, binary(), map()} | {:error, term()}