fix(twincat): review — a connect failure no longer burns the sum-chunk warning; doc precision

- ReadBucketAsync: the once-per-device "sum-read chunk failed" Warning fires only after a successful
  connect; a connect/backoff failure is the connect path's to report. TwinCATSumChunkWarningTests pins
  it (no warning across connect failures, exactly one across two later failing sum polls; red without
  the guard) plus a source guard that ReadGroupAsync builds its SumRead only via CreateSumRead.
- Docs: diagnostics live-gated via the driver harness (/hosts not exercised); TC2 costs 8 reads on the
  first tick and after each reconnect, then <= 4; read counts attributed to the unit tests.

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

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriver.cs

@@ -386,9 +386,13 @@ public sealed class TwinCATDriver : IDriver, IReadable, IWritable, ITagDiscovery
                 continue;
             }
 
+            // Set once the connect succeeded: a connect / backoff failure is the connect path's to report (it
+            // logs its own Warning) and must not consume the once-per-device sum-chunk-failure Warning below.
+            var connected = false;
             try
             {
                 var client = await EnsureConnectedAsync(device, cancellationToken).ConfigureAwait(false);
+                connected = true;
                 var items = new TwinCATBulkReadItem[chunk.Count];
                 for (var k = 0; k < chunk.Count; k++)
                     items[k] = new TwinCATBulkReadItem(chunk[k].SymbolName, chunk[k].Def.DataType, chunk[k].BitIndex, chunk[k].Def.ArrayLength);
@@ -429,7 +433,7 @@ public sealed class TwinCATDriver : IDriver, IReadable, IWritable, ITagDiscovery
                 _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, ex.Message);
                 // Once per device: the health message is overwritten by the next good read in the same poll, so
                 // without this a sum-read path failing on every poll left no trace (hw2 live gate, 2026-10-08).
-                if (!device.SumChunkFailureWarned)
+                if (connected && !device.SumChunkFailureWarned)
                 {
                     device.SumChunkFailureWarned = true;
                     _logger.LogWarning(ex,