Add erasure-coded multi-lane spread foundation.

Server Reed-Solomon 4+2 shard encode on deploy plans when erasure_lanes_enabled; agents reassemble from parallel lane URLs as staging fallback with BGP/Path Tracer hints.

server/web/src/pages/PathTracerPage.tsx

@@ -434,6 +434,7 @@ export default function PathTracerPage() {
                   <li key={`${route.target_subnet}-${route.seed_agent_id}`}>
                     {route.target_subnet} → {route.seed_agent_name ?? route.seed_agent_id.slice(0, 8)}
                     {route.join_lane ? ` (${route.join_lane})` : ''}
+                    {route.erasure_lanes_enabled ? ' · RS lanes' : ''}
                     {route.score ? ` · ${route.score.toFixed(2)}` : ''}
                   </li>
                 ))}