fix(mtconnect): a delivered /sample chunk clears an unanswered-write degradation (review r2 N-5)

Coordinator ruling: a delivered chunk proves the Agent is alive (the analogue of
OpcUaClient's good keep-alive), so TryMarkStreamDelivered - generation-checked
under _subscriptionLock - also clears _writeDegraded. The degradation no longer
sticks forever on a subscription-only driver. Test: unanswered write -> Degraded,
stays Degraded with nothing else happening (positive control), the next chunk ->
Healthy; red with the clear removed. Round-2 doc text corrected.

Refs #743

Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect/MTConnectDriver.cs

@@ -371,9 +371,10 @@ public sealed partial class MTConnectDriver
 
     /// <summary>
     ///     Whether the last Agent write got no reply (review I-3 / coordinator ruling: an unanswered write degrades).
-    ///     Latched until the next answered <c>/current</c> read or answered write (<see cref="RecordReadAnswered"/> /
-    ///     <see cref="RecordWriteAnswered"/>). <b>Neither a <c>/sample</c> chunk nor a live-index read clears it</b>, so on
-    ///     a subscription-only driver one timed-out write keeps the driver Degraded until the next write (r1 N-5).
+    ///     Cleared by the next answered <c>/current</c> read, answered write (<see cref="RecordReadAnswered"/> /
+    ///     <see cref="RecordWriteAnswered"/>) or delivered <c>/sample</c> chunk of the current session
+    ///     (<see cref="TryMarkStreamDelivered"/>, r2 N-5) — so it cannot stick on a subscription-only driver. A read served
+    ///     from the live index does not clear it directly; the chunks behind that index do.
     /// </summary>
     private volatile bool _writeDegraded;
 
@@ -856,9 +857,8 @@ public sealed partial class MTConnectDriver
 
         // The read path is answering (from a stream that is delivering), so a degradation a failed /current left
         // behind is no longer true of it. The stamp stays the pump's — this read heard nothing from the Agent itself.
-        // Clears only the READ degradation (r1 N-5): an index read is the stream's evidence, not an Agent answer to a
-        // request — the same evidence a /sample chunk carries, which by the I-3 ruling does not clear an unanswered
-        // write either.
+        // Clears only the READ degradation: an index read is derived from the stream's evidence — the chunks that fed the
+        // index are what clear an unanswered write (TryMarkStreamDelivered, r2 N-5).
         if (_readDegraded)
         {
             _readDegraded = false;
@@ -2751,6 +2751,11 @@ public sealed partial class MTConnectDriver
 
             _streamDegraded = false;
             _pumpLive = true;
+
+            // r2 N-5 (coordinator): a delivered chunk proves the Agent is alive — the MTConnect analogue of OpcUaClient's
+            // good keep-alive clearing a read/write Degraded — so it also clears an unanswered-write degradation, which
+            // otherwise stuck forever on a subscription-only driver. Generation-checked with the rest (above).
+            _writeDegraded = false;
             PublishHealthy(DateTime.UtcNow);
 
             return true;