Auto: focas-f4a — write infrastructure + per-tag opt-in

Closes #268

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

@@ -73,7 +73,10 @@ public static class FocasDriverFactoryExtensions
                     Address: t.Address ?? throw new InvalidOperationException(
                         $"FOCAS tag '{t.Name}' in '{driverInstanceId}' missing Address"),
                     DataType: ParseDataType(t.DataType, t.Name!, driverInstanceId),
-                    Writable: t.Writable ?? true,
+                    // Per-tag Writable defaults to false post-F4-a (issue #268). A config-DB row
+                    // with Writable null means "not opted in" — operators must explicitly flip
+                    // the flag per tag before writes flow.
+                    Writable: t.Writable ?? false,
                     WriteIdempotent: t.WriteIdempotent ?? false))]
                 : [],
             Probe = new FocasProbeOptions
@@ -83,6 +86,13 @@ public static class FocasDriverFactoryExtensions
                 Timeout = TimeSpan.FromMilliseconds(dto.Probe?.TimeoutMs ?? 2_000),
             },
             Timeout = TimeSpan.FromMilliseconds(dto.TimeoutMs ?? 2_000),
+            // Driver-level write opt-in (issue #268, plan PR F4-a). Default false — config rows
+            // that omit the section keep the safer-by-default read-only posture; flipping it on
+            // requires an explicit deployment-time choice.
+            Writes = new FocasWritesOptions
+            {
+                Enabled = dto.Writes?.Enabled ?? false,
+            },
         };
 
         var clientFactory = BuildClientFactory(dto, driverInstanceId);
@@ -170,6 +180,12 @@ public static class FocasDriverFactoryExtensions
         public List<FocasDeviceDto>? Devices { get; init; }
         public List<FocasTagDto>? Tags { get; init; }
         public FocasProbeDto? Probe { get; init; }
+        public FocasWritesDto? Writes { get; init; }
+    }
+
+    internal sealed class FocasWritesDto
+    {
+        public bool? Enabled { get; init; }
     }
 
     internal sealed class FocasDeviceDto