feat(hosts): driver diagnostics channel on /hosts, S7 negotiated PDU size first (#640)

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Drivers/DriverInstanceActor.cs

@@ -1153,6 +1153,28 @@ public sealed class DriverInstanceActor : ReceiveActor, IWithTimers
         }
     }
 
+    /// <summary>Bounded driver diagnostics, or null when the driver is not an <see cref="IDriverDiagnostics"/>
+    /// (Gitea #640). Pulled fresh and called UNGUARDED for the same reasons as
+    /// <see cref="CurrentSkippedTags"/>; a throwing implementation loses only the diagnostics.</summary>
+    private IReadOnlyDictionary<string, string>? CurrentDiagnostics()
+    {
+        if (_driver is not IDriverDiagnostics diag) return null;
+        try
+        {
+            return DriverDiagnostics.Bound(diag.GetDiagnostics());
+        }
+        catch (Exception ex)
+        {
+            _log.Warning(ex, "DriverInstance {Id}: GetDiagnostics threw during health publish; omitting diagnostics", _driverInstanceId);
+            return null;
+        }
+    }
+
+    /// <summary>Value digest of the diagnostics bag for the publish fingerprint. Null in, null out. The bag is
+    /// already in ordinal key order (<see cref="DriverDiagnostics.Bound"/>), so equal facts digest equally.</summary>
+    private static string? DiagnosticsDigest(IReadOnlyDictionary<string, string>? d) =>
+        d is null ? null : string.Join('|', d.Select(kv => $"{kv.Key}={kv.Value}"));
+
     /// <summary>Records the driver's rediscovery raise and re-publishes health so the signal reaches the
     /// AdminUI promptly rather than waiting for the next 30 s heartbeat.
     /// <para><b>Advisory only.</b> The served address space is deliberately NOT rebuilt: v3 authors raw tags
@@ -1357,6 +1379,7 @@ public sealed class DriverInstanceActor : ReceiveActor, IWithTimers
             var errorCount = ErrorCount5Min();
             var hostStatuses = CurrentHostStatuses();
             var skippedTags = CurrentSkippedTags();
+            var diagnostics = CurrentDiagnostics();
             // _rediscoveryNeededUtc is PART OF THE FINGERPRINT on purpose. A rediscovery raise on an
             // otherwise-unchanged Healthy driver leaves (state, lastSuccess, lastError, errorCount)
             // identical, so without it the dedup below would swallow the very publish that carries the
@@ -1375,14 +1398,20 @@ public sealed class DriverInstanceActor : ReceiveActor, IWithTimers
             // is identical to a clean driver's and the publish that carries the signal is exactly the one
             // dedup would swallow. Flattened to a STRING for the same reason as the host digest — the
             // report holds an IReadOnlyList, which a tuple compares by reference.
+            //
+            // The diagnostics digest (Gitea #640) is the fourth instance: a diagnostic fact (S7's PDU size
+            // changing across a reconnect) moves none of the other components, so without it the publish
+            // that carries it is deduped away. Flattened to a STRING - the bag is a dictionary, which a
+            // tuple compares by reference.
             var fingerprint = (health.State, health.LastSuccessfulRead, health.LastError, errorCount,
-                _rediscoveryNeededUtc, HostStatusDigest(hostStatuses), SkippedTagDigest(skippedTags));
+                _rediscoveryNeededUtc, HostStatusDigest(hostStatuses), SkippedTagDigest(skippedTags),
+                DiagnosticsDigest(diagnostics));
             if (_lastPublishedFingerprint is { } prev && prev.Equals(fingerprint))
                 return observed;
             _lastPublishedFingerprint = fingerprint;
             _healthPublisher.Publish(
                 _clusterId, _driverInstanceId, health, errorCount, _rediscoveryNeededUtc, _rediscoveryReason,
-                hostStatuses, skippedTags);
+                hostStatuses, skippedTags, diagnostics);
         }
         catch (Exception ex)
         {
@@ -1418,7 +1447,8 @@ public sealed class DriverInstanceActor : ReceiveActor, IWithTimers
 
     /// <summary>Fingerprint of the last <see cref="PublishHealthSnapshot"/> call; null until first publish.</summary>
     private (DriverState State, DateTime? LastSuccess, string? LastError, int ErrorCount,
-        DateTime? RediscoveryNeededUtc, string? HostStatusDigest, string? SkippedTagDigest)? _lastPublishedFingerprint;
+        DateTime? RediscoveryNeededUtc, string? HostStatusDigest, string? SkippedTagDigest,
+        string? DiagnosticsDigest)? _lastPublishedFingerprint;
 
     /// <inheritdoc />
     protected override void PostStop()