review(Driver.AbCip.Contracts): first review; document IsArray/Writable contract

First review at 7286d320. 6 findings (doc fixes -005/-006 resolved; -001 writable-hardcode,
-002 Dt-units doc, -003 dead [Display] attrs, -004 parser test gap left Open). Surfaced
cross-module: AbCipTagDto/AbCipMemberDto in the factory drop ElementCount/IsArray.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip.Contracts/AbCipDriverOptions.cs

@@ -142,7 +142,8 @@ public sealed record AbCipDeviceOptions(
 ///     had <c>isArray:true</c> (with <c>arrayLength &gt;= 1</c>); the tag discovers as an OPC UA
 ///     array node (<c>IsArray</c> + <c>ArrayDim</c>) and reads as a typed CLR array — even when
 ///     <paramref name="ElementCount"/> is 1 (a valid 1-element array). <c>ElementCount</c> alone
-///     cannot carry this because a scalar and a 1-element array both have a count of 1.</param>
+///     cannot carry this because a scalar and a 1-element array both have a count of 1.
+///     Ignored for <see cref="AbCipDataType.Structure"/>.</param>
 public sealed record AbCipTagDefinition(
     string Name,
     string DeviceHostAddress,

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip.Contracts/AbCipEquipmentTagParser.cs

@@ -12,6 +12,12 @@ public static class AbCipEquipmentTagParser
     /// <param name="reference">The equipment tag's TagConfig JSON (also used as the def identity).</param>
     /// <param name="def">The transient definition when parsing succeeds.</param>
     /// <returns><see langword="true"/> when <paramref name="reference"/> is an AbCip TagConfig object.</returns>
+    /// <remarks>
+    ///     The produced <see cref="AbCipTagDefinition.Writable"/> is always <c>true</c>: the
+    ///     TagConfig JSON format for equipment tags does not carry a writability field, so the
+    ///     PLC's ExternalAccess attribute is the effective write gate. Operators who need a
+    ///     read-only OPC UA surface must rely on the PLC's ExternalAccess rejecting the write.
+    /// </remarks>
     public static bool TryParse(string reference, out AbCipTagDefinition def)
     {
         def = null!;