add function to apply mutations over grpc

proto/control_plane.proto

@@ -4,6 +4,8 @@ package messages;
 
 option go_package = "git.k6n.net/go-cart-actor/proto;messages";
 
+import "messages.proto";
+
 // -----------------------------------------------------------------------------
 // Control Plane gRPC API
 // -----------------------------------------------------------------------------
@@ -65,6 +67,16 @@ message ExpiryAnnounce {
   repeated uint64 ids = 2;
 }
 
+message ApplyRequest {
+
+    uint64 id = 1;
+    repeated Mutation messages = 2;
+}
+
+message ApplyResult {
+  bool accepted = 1;
+}
+
 // ControlPlane defines cluster coordination and ownership operations.
 service ControlPlane {
   // Ping for liveness; lightweight health signal.
@@ -80,6 +92,7 @@ service ControlPlane {
 
   // Ownership announcement: first-touch claim broadcast (idempotent; best-effort).
   rpc AnnounceOwnership(OwnershipAnnounce) returns (OwnerChangeAck);
+  rpc Apply(ApplyRequest) returns (ApplyResult);
 
   // Expiry announcement: drop remote ownership hints when local TTL expires.
   rpc AnnounceExpiry(ExpiryAnnounce) returns (OwnerChangeAck);