Don't send a full update when a subnet router goes up or down; the gated
policy change already recomputes peers and is a smaller payload.
Updates #3293
Connect and Disconnect appended change.PolicyChange() on every reconnect. PolicyChange sets RequiresRuntimePeerComputation, so the batcher rebuilt a full netmap (packet filters, SSH policy, peer serialization) for every connected node — O(N) per reconnect, O(N^2) on a restart storm. On a small VM this saturated CPU after the v0.28 -> v0.29 upgrade.
Emit it only when the node's online state changes what peers compute: subnet routers, relay targets, and via targets. An ordinary reconnect now sends just the lightweight online/offline peer patch. Relay and via targets still recompute, so peers drop a stale PeerRelay allocation when a relay goes offline.
Fixes#3293
