Dynamic supervisor for this service's macula_subscriber children (piece D, PLAN_HECATE_OM_MESH_WRAPPERS.md).

Starts empty; hecate_om_pubsub_subscriptions adds/removes one child per declared {Topic, HandlerMod, Args}, keyed by Topic so it can be looked up and torn down individually. restart => transient: a genuine subscription loss (macula_subscriber turns {macula_event_gone, ...} into {stop, Reason, State}) gets an ordinary supervisor restart -- "let it crash" is the whole reconnect story here, since macula_client_replay:subs_to/2 already replays a subscription across an ordinary link respawn with no message to this process at all (confirmed live, macula_link_respawn_replay_tests.erl in macula-io/macula). An explicit terminate_child (no longer desired) is not a crash and triggers no restart.