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_test.go

@@ -0,0 +1,19 @@
+package config
+
+import "testing"
+
+func TestFleetTorrentEnabled(t *testing.T) {
+	if FleetTorrentEnabled(RuntimeConfig{}) {
+		t.Fatal("expected false")
+	}
+	if !FleetTorrentEnabled(RuntimeConfig{BuiltinConfig: BuiltinConfig{FleetTorrentEnabled: true}}) {
+		t.Fatal("expected true")
+	}
+}
+
+func TestIsSubnetPrimarySeeder(t *testing.T) {
+	cfg := RuntimeConfig{BuiltinConfig: BuiltinConfig{SubnetPrimarySeeder: true}}
+	if !cfg.IsSubnetPrimarySeeder() {
+		t.Fatal("expected primary")
+	}
+}