perf(comms+audit): close phase-2 residuals — direct ingest path, monotonic timeouts, synthetic probe, not-reporting set, cursor-exact audit pull

src/ZB.MOM.WW.ScadaBridge.Communication/Protos/central_control.proto

@@ -244,4 +244,12 @@ message HeartbeatDto {
   string node_hostname = 2;
   bool is_active = 3;
   google.protobuf.Timestamp timestamp = 4;
+  // Additive (field 5): true marks a SYNTHETIC heartbeat that carries no liveness
+  // meaning — today only CentralChannelProvider's failback probe, which reuses this
+  // RPC purely to test whether the preferred central endpoint answers again. Central
+  // must skip ALL liveness/health bookkeeping for such a heartbeat: it does not come
+  // from a site node's heartbeat timer, and treating it as one would keep a site
+  // looking alive on the health dashboard from the probe alone. proto3 defaults it to
+  // false, so a pre-existing site's heartbeat is a real one, as before.
+  bool synthetic = 5;
 }