refactor once again

proto/control_plane.proto

@@ -52,6 +52,13 @@ message ClosingNotice {
   string host = 1;
 }
 
+// OwnershipAnnounce broadcasts first-touch ownership claims for cart IDs.
+// First claim wins; receivers SHOULD NOT overwrite an existing different owner.
+message OwnershipAnnounce {
+  string host = 1;               // announcing host
+  repeated string cart_ids = 2;   // newly claimed cart ids
+}
+
 // ControlPlane defines cluster coordination and ownership operations.
 service ControlPlane {
   // Ping for liveness; lightweight health signal.
@@ -65,6 +72,9 @@ service ControlPlane {
 
   // ConfirmOwner RPC removed (was legacy ownership acknowledgement; ring-based ownership now authoritative)
 
+  // Ownership announcement: first-touch claim broadcast (idempotent; best-effort).
+  rpc AnnounceOwnership(OwnershipAnnounce) returns (OwnerChangeAck);
+
   // Closing announces graceful shutdown so peers can proactively adjust.
   rpc Closing(ClosingNotice) returns (OwnerChangeAck);
 }