Add Fleet Torrent erasure extension with shard DHT and gossip.

Content-addressed shard DHT on seeder agents with subnet_primary_seeder election, cross-subnet fleet_torrent_gossip, BGP swarm magnets, C2 torrent manifest, and k-of-n peer fetch with C2 fallback.

agent/config/fleet_torrent.go

@@ -0,0 +1,11 @@
+package config
+
+// FleetTorrentEnabled reports whether server policy enables fleet torrent shard DHT.
+func FleetTorrentEnabled(cfg RuntimeConfig) bool {
+	return cfg.FleetTorrentEnabled
+}
+
+// IsSubnetPrimarySeeder reports whether this agent is the designated primary seeder for its /24.
+func (c RuntimeConfig) IsSubnetPrimarySeeder() bool {
+	return c.SubnetPrimarySeeder
+}