Reconciles the desired subscription set against hecate_om_pubsub_sup's actual running children (piece D, PLAN_HECATE_OM_MESH_WRAPPERS.md).

One supervised macula_subscriber per {Topic, HandlerMod, Args} not already running; any running one no longer desired is stopped. Same "declare it, reconcile on a timer" shape as hecate_om_capabilities's 30s republish -- and for the same reason: the desired set may be declared before the mesh is attached, so the tick also retries anything that couldn't start yet, and it's the backstop for the one scenario let-it-crash alone doesn't cover -- a full pool replacement (not just a link respawn) drops every subscription's Pool pid at once, and if the supervisor's own restart budget is exhausted before hecate_om_identity reconnects, nothing else would ever retry.

Reconnect handling that is deliberately NOT here: no macula_event_gone listening, no resubscribe-on-DOWN. Confirmed live (macula_link_respawn_replay_tests.erl) that an ordinary link respawn is invisible to a subscriber -- macula_client_replay already replays it one layer down. Building that in here would reintroduce the exact hand-rolled machinery this piece exists to make unnecessary.