Config-driven event publisher for a live packing feed. No callbacks: when ExBoxPacker.Config.broadcast/0 is set (and absinthe is loaded), each event is published via Absinthe.Subscription.publish(endpoint, event, [{field, topic}]).

A nil topic or absent config makes every function a cheap no-op, so packing outside a broadcasting context is unaffected. A :publisher 2-arity fn in the broadcast config overrides the Absinthe path (used in tests).