The cross-domain A2R bridge is forced instead of demand-driven #1570

The A2R bridge forced from the domain bridge rule config (#1558) is unconditional because gossip is confined to the agent's own DDS domain, leaving an agent no evidence of a subscriber in to_domain. Two ways to supply that evidence: (A) give the agent one rclpy context per domain named in the config, which costs an executor thread per domain and needs the kmod's per-(ns, domain) singleton claim reworked; or (B) move gossip to a single well-known discovery domain that every agent joins, which also collapses the agent back to one per namespace, since the per-domain split exists only so gossip can ride the application's own domain. (B) is the smaller change but puts a participant on a shared domain, which conflicts with using domains for isolation.