Auto: focas-f4c — pmc_wrpmcrng with bit-level RMW

Closes #270

src/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/FocasDriverOptions.cs

@@ -83,6 +83,20 @@ public sealed record FocasWritesOptions
     ///     gate requires <c>WriteOperate</c> group membership.</para>
     /// </summary>
     public bool AllowMacro { get; init; } = false;
+
+    /// <summary>
+    ///     Issue #270, plan PR F4-c — granular kill-switch for <c>pmc_wrpmcrng</c> PMC
+    ///     range writes (and the bit-level read-modify-write that wraps it). Default
+    ///     <c>false</c>: PMC is ladder working memory — a mistargeted bit can move
+    ///     motion, latch a feedhold, or flip a safety interlock. Even with
+    ///     <see cref="Enabled"/> on and a tag's <see cref="FocasTagDefinition.Writable"/>
+    ///     flag flipped on, PMC writes stay locked until this third opt-in fires.
+    ///     <para>Server-layer ACL: PMC tags surface
+    ///     <see cref="Core.Abstractions.SecurityClassification.Operate"/> so the OPC UA
+    ///     gate requires <c>WriteOperate</c> group membership; this flag is the driver-
+    ///     level kill switch the operator team can flip without a redeploy.</para>
+    /// </summary>
+    public bool AllowPmc { get; init; } = false;
 }
 
 /// <summary>