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/internal/api/spreadrouter_bridge.go

@@ -99,6 +99,7 @@ func buildSpreadRouterInput(hub *WSHub, sessions []*TraceSession, targetSubnets
 	}
 
 	in.LaneSuccess = collectLaneSuccessStats(hub)
+	in.ErasureLanesEnabled = hub.serverPolicySnapshot().ErasureLanesEnabled
 	return in
 }