Realm-scoped PubSub state + dispatch (Part 6 §6).

Holds the topic-to-subscriber index for one realm and converts incoming SUBSCRIBE / UNSUBSCRIBE / EVENT frames into local state mutations + delivery instructions. The wire layer that actually transmits frames lives elsewhere — typically hecate_plumtree for intra-realm fan-out.

Pipeline

• Local subscribe — subscribe/3 adds the subscriber to the topic's set. The wrapper builds a SUBSCRIBE frame for upstream propagation if needed.
• Local publish — build_event/3 takes a PUBLISH spec and produces a signed EVENT frame the wrapper hands to Plumtree for fan-out. The publisher signs the EVENT once; intermediate hops do NOT re-sign, so every subscriber can verify authenticity end-to-end (Part 6 §6.4).
• Receive EVENT — deliver_event/2 returns the list of local subscribers whose subscription matches the event's topic + realm. The wrapper notifies each via the application channel.
• Receive SUBSCRIBE / UNSUBSCRIBE — process/3 updates local state.

State is per realm: an instance handles one realm's topics only. Cross-realm leakage is impossible — the realm is baked into the state and every dispatch checks it.

Reference: plans/PLAN_MACULA_V2_PART6_PROTOCOL.md §6; plans/PLAN_PHASE_5_BREAKDOWN.md Session 5.5.