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/Grpc/CentralChannelProvider.cs

@@ -38,6 +38,15 @@ namespace ZB.MOM.WW.ScadaBridge.Communication.Grpc;
 /// </remarks>
 public sealed class CentralChannelProvider : IDisposable
 {
+    /// <summary>
+    /// Hostname stamped on the failback probe's heartbeat, so a central-side log line names it
+    /// for what it is. It is a LABEL only — the machine-readable contract is
+    /// <see cref="ZB.MOM.WW.ScadaBridge.Commons.Messages.Health.HeartbeatMessage.Synthetic"/>
+    /// (<c>HeartbeatDto.synthetic</c>, proto field 5), which central keys its skip on. Never
+    /// filter on this string.
+    /// </summary>
+    public const string SyntheticProbeHostname = "failback-probe";
+
     private static readonly TimeSpan DefaultBackoffBase = TimeSpan.FromSeconds(1);
     private static readonly TimeSpan DefaultBackoffCap = TimeSpan.FromSeconds(60);
     private static readonly TimeSpan DefaultProbeDeadline = TimeSpan.FromSeconds(5);
@@ -202,9 +211,16 @@ public sealed class CentralChannelProvider : IDisposable
                 new HeartbeatDto
                 {
                     SiteId = _siteId,
-                    NodeHostname = "failback-probe",
+                    NodeHostname = SyntheticProbeHostname,
                     IsActive = false,
                     Timestamp = Timestamp.FromDateTimeOffset(DateTimeOffset.UtcNow),
+                    // Explicit contract, not a hostname convention: central skips ALL
+                    // liveness/health bookkeeping for a synthetic heartbeat. Without it the
+                    // probe would stamp the site's last-seen time on the central health
+                    // aggregator every backoff tick, so a site whose real heartbeats had
+                    // stopped could still look alive purely because its transport layer was
+                    // probing for failback.
+                    Synthetic = true,
                 },
                 deadline: DateTime.UtcNow.Add(_probeDeadline)).ConfigureAwait(false);