some cleanup and annonce expiry

proto/control_plane.proto

@@ -59,6 +59,12 @@ message OwnershipAnnounce {
   repeated uint64 cart_ids = 2;   // newly claimed cart ids
 }
 
+// ExpiryAnnounce broadcasts that a host evicted the provided cart IDs.
+message ExpiryAnnounce {
+  string host = 1;
+  repeated uint64 cart_ids = 2;
+}
+
 // ControlPlane defines cluster coordination and ownership operations.
 service ControlPlane {
   // Ping for liveness; lightweight health signal.
@@ -75,6 +81,9 @@ service ControlPlane {
   // Ownership announcement: first-touch claim broadcast (idempotent; best-effort).
   rpc AnnounceOwnership(OwnershipAnnounce) returns (OwnerChangeAck);
 
+  // Expiry announcement: drop remote ownership hints when local TTL expires.
+  rpc AnnounceExpiry(ExpiryAnnounce) returns (OwnerChangeAck);
+
   // Closing announces graceful shutdown so peers can proactively adjust.
   rpc Closing(ClosingNotice) returns (OwnerChangeAck);
 }