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

Closes #255

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

@@ -45,7 +45,14 @@ public static class AbLegacyDriverFactoryExtensions
                     DeviceName: d.DeviceName,
                     // PR 9 — per-device timeout / retry overrides. Device-level wins over driver-wide.
                     Timeout: d.TimeoutMs is int devMs ? TimeSpan.FromMilliseconds(devMs) : null,
-                    Retries: d.Retries))]
+                    Retries: d.Retries,
+                    // PR ablegacy-12 / #255 — auto-demote knobs.
+                    Demote: d.Demote is null ? null : new AbLegacyDemoteOptions(
+                        FailureThreshold: d.Demote.FailureThreshold ?? 3,
+                        DemoteFor: d.Demote.DemoteForMs is int demMs
+                            ? TimeSpan.FromMilliseconds(demMs)
+                            : null,
+                        Enabled: d.Demote.Enabled ?? true)))]
                 : [],
             Tags = dto.Tags is { Count: > 0 }
                 ? [.. dto.Tags.Select(t => new AbLegacyTagDefinition(
@@ -209,6 +216,26 @@ public static class AbLegacyDriverFactoryExtensions
         ///     <c>null</c> at both levels = single attempt.
         /// </summary>
         public int? Retries { get; init; }
+
+        /// <summary>
+        ///     PR ablegacy-12 / #255 — optional per-device auto-demote knobs. <c>null</c>
+        ///     means "use the documented defaults" (<c>FailureThreshold=3</c>,
+        ///     <c>DemoteFor=30s</c>, <c>Enabled=true</c>) — the driver still demotes by
+        ///     default. Set <c>Enabled=false</c> in the JSON to opt out entirely.
+        /// </summary>
+        public AbLegacyDemoteDto? Demote { get; init; }
+    }
+
+    /// <summary>
+    ///     PR ablegacy-12 / #255 — JSON DTO for the auto-demote knobs. Times are
+    ///     ms-suffixed for consistency with the rest of the driver config (TimeoutMs,
+    ///     IntervalMs).
+    /// </summary>
+    internal sealed class AbLegacyDemoteDto
+    {
+        public int? FailureThreshold { get; init; }
+        public int? DemoteForMs { get; init; }
+        public bool? Enabled { get; init; }
     }
 
     internal sealed class AbLegacyTagDto