Tenant-scoped PubSub fan-out for operator dashboard live events.

Publishes incremental trace deltas to scoria:runs:{tenant_id}. Tracks seen trace_id values per BEAM node in ETS (:scoria_observe_operator_broadcast_trace_seen) so {:trace_opened, _} is emitted only on the first span stop for a trace on this node.

Missing tenant_id drops broadcast (fail closed) with a debug log — no global topic fallback.