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.

server/web/src/components/Fleet/AccessDepthPanel.tsx

@@ -188,6 +188,9 @@ export default function AccessDepthPanel({ agent, diagnostics }: Props) {
           </span>
         )}
         {!policyLoaded && <span className="access-depth-muted">loading policy…</span>}
+        <Link to="/oath" className="access-depth-oath-link" title="Immutable operator accountability ledger">
+          Oath
+        </Link>
       </div>
 
       <div className="access-depth-grid">