Auto: ablegacy-12 — auto-demote on comm failure

Closes #255

src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriverOptions.cs

@@ -41,7 +41,39 @@ public sealed record AbLegacyDeviceOptions(
     AbLegacyPlcFamily PlcFamily = AbLegacyPlcFamily.Slc500,
     string? DeviceName = null,
     TimeSpan? Timeout = null,
-    int? Retries = null);
+    int? Retries = null,
+    AbLegacyDemoteOptions? Demote = null);
+
+/// <summary>
+///     PR ablegacy-12 / #255 — auto-demote knobs. After
+///     <see cref="FailureThreshold"/> consecutive read / probe failures the driver
+///     marks the device <c>Demoted</c> for <see cref="DemoteFor"/>; reads against
+///     a demoted device short-circuit with <c>BadCommunicationError</c> instead
+///     of dispatching through libplctag, so one slow PLC can't starve faster
+///     peers sharing the same driver. A successful probe clears the demotion
+///     early; a successful read just resets the consecutive-failure counter
+///     without leaving the demoted window.
+/// </summary>
+/// <param name="FailureThreshold">Consecutive read or probe failures that trip
+///   the demotion. Default <c>3</c>.</param>
+/// <param name="DemoteFor">Cool-down window before reads are dispatched again
+///   without a successful probe in between. Default <c>30s</c>.</param>
+/// <param name="Enabled">When <c>false</c> the failure tally still ticks but the
+///   driver never sets the demoted window — useful when an operator wants the
+///   diagnostic counters without the throttling behaviour.</param>
+public sealed record AbLegacyDemoteOptions(
+    int FailureThreshold = 3,
+    TimeSpan? DemoteFor = null,
+    bool Enabled = true)
+{
+    /// <summary>
+    ///     Effective demote window. Records can't have <c>TimeSpan</c> defaults
+    ///     because <c>TimeSpan.FromSeconds(30)</c> isn't a compile-time constant;
+    ///     callers that pass <c>null</c> get the documented 30-second default
+    ///     here.
+    /// </summary>
+    public TimeSpan EffectiveDemoteFor => DemoteFor ?? TimeSpan.FromSeconds(30);
+}
 
 /// <summary>
 ///     One PCCC-backed OPC UA variable. <c>Address</c> is the canonical PCCC file-address