docs(r2-06): note WriteLiveValues Quality is a documented-dead input (0.2.0 C-002); keep 192 populated (06/C-7)

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway/Recorder/GatewayHistorianValueWriter.cs

@@ -24,6 +24,17 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Recorder;
 ///         A success ack OR a store-forward-queued ack maps to <c>true</c>: a value the gateway
 ///         durably queued must not be re-drained.
 ///     </para>
+///     <para>
+///         <b>Quality is a documented-dead input</b> on the <c>WriteLiveValues</c> SQL path
+///         (HistorianGateway 0.2.0 Contracts C-002): <c>HistorianLiveValue.Quality</c> /
+///         <c>QualityDetail</c> are server-managed columns — any value sent is accepted on the wire and
+///         then silently discarded. We populate <c>Quality</c> anyway (see the mapping below): proto3
+///         transmits <c>0</c> when a field is left unset, and <c>0</c> is "Bad" in OPC-DA terms, so
+///         <em>not</em> setting it would send a <em>wrong</em> quality to any future gateway version that
+///         starts honoring the field — whereas the recorder's <c>192</c> ("Good") is the correct value.
+///         <b>Do NOT extend the recorder to capture real node quality expecting it to persist</b> — it
+///         will not, until the gateway honors the field.
+///     </para>
 /// </remarks>
 public sealed class GatewayHistorianValueWriter : IHistorianValueWriter, IAsyncDisposable
 {
@@ -60,6 +71,8 @@ public sealed class GatewayHistorianValueWriter : IHistorianValueWriter, IAsyncD
                 var live = new HistorianLiveValue
                 {
                     NumericValue = value.Value,
+                    // Documented-dead input on the WriteLiveValues SQL path (0.2.0 Contracts C-002 —
+                    // server-managed, silently discarded). Populated deliberately; see the class <remarks>.
                     Quality = value.Quality,
                 };