The built-in send-pipeline step that records each outgoing message in the Amarula.RetryCache, so it can be re-encrypted and resent if the recipient asks for a retry. Attached by default (see Amarula.Conn); a side-effect step that never transforms or halts.

Expects the send ctx to carry :retry_cache (scope), :profile, :msg_id, :message, :to, and (optionally) :stanza_attrs.