statement-store: add event-fed v2 peer topology

[DenzelPenzel]

Summary

This PR adds the initial statement-store v2 peer topology wiring. #11933

It introduces backend-neutral network events for peer discovery and identify
metadata:

• Event::PeerDiscovered(PeerId)
• Event::PeerIdentified { peer, supported_protocols }

Both libp2p and litep2p backends now emit these events from their existing
discovery/routing-table and identify paths.

The statement handler consumes these events and updates a new
PeersTopology module. The topology keeps a local view of statement-capable
peers learned from network discovery, identify metadata, and statement
notification connections.

Design notes

PeersTopology intentionally computes closest peers over the locally learned
peer set. It does not perform topic-specific Kademlia lookups.

The topology currently supports:

• tracking seen / identified statement-capable peers;
• tracking statement notification connection state;
• filtering DHT-eligible peers to full V2 statement peers;
• computing local DHT storage affinity;
• selecting K closest connected DHT routing targets;
• selecting explicit-affinity connection candidates for topics

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
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