Realtime audience dispatch

kickside.contract:realtime_batch.notify accepts user_ids (one to 128), topic and optional data. It authorizes the whole audience, deduplicates users, and invokes the selected kickside.contract:realtime.notify owner concurrently with its existing scalar user_id input. Every response completes before the batch reports success; native errors retain their metadata.

This package has no host requirements and depends only on kickside/contract. Relay remains an implementation of the scalar contract. Offline clients reconcile their state on reconnect; realtime dispatch creates no durable delivery promise.

The core leaf owns dispatch and failure semantics. The host selects the scalar contract binding. An unbound scalar capability returns {success=true, notified=0, status="not_supported"}; failures from a bound owner propagate as native errors.