perf(abcip): cache the declaration-only UDT layout per member list

P11: AbCipUdtReadPlanner called AbCipUdtMemberLayout.TryBuild(parent.Members)
for every member reference on every poll - O(members^2) per parent per tick,
a dictionary allocation each time. GetOrBuild caches the layout (including a
"not groupable" verdict) in a ConditionalWeakTable keyed by the member list's
identity, which is immutable for the life of the tag table; a re-init's new
lists replace the entries and the old ones are collected with them.

Refs #737
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip/AbCipUdtMemberLayout.cs

@@ -25,6 +25,30 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.AbCip;
 /// </remarks>
 public static class AbCipUdtMemberLayout
 {
+    /// <summary>Layouts already computed, keyed by the parent definition's member list (reference identity).</summary>
+    private static readonly System.Runtime.CompilerServices.ConditionalWeakTable<IReadOnlyList<AbCipStructureMember>, LayoutBox> Cache = new();
+
+    /// <summary>Holds a computed layout, including a "not groupable" (<c>null</c>) verdict.</summary>
+    private sealed class LayoutBox(IReadOnlyDictionary<string, int>? offsets)
+    {
+        public IReadOnlyDictionary<string, int>? Offsets { get; } = offsets;
+    }
+
+    /// <summary>
+    ///     <see cref="TryBuild"/>, computed once per member list (review 07 P11: the read planner rebuilt the layout —
+    ///     a dictionary allocation and an O(members) walk — for every member reference on every poll, O(members²) per
+    ///     parent per tick). A parent definition's member list is immutable for the life of the tag table that holds
+    ///     it, so caching by list identity is exact; a re-init builds new lists and the old entries are collected with
+    ///     them (<see cref="System.Runtime.CompilerServices.ConditionalWeakTable{TKey,TValue}"/>).
+    /// </summary>
+    /// <param name="members">The parent definition's declared members.</param>
+    /// <returns>The member offsets, or <c>null</c> when the members cannot be laid out by declaration.</returns>
+    public static IReadOnlyDictionary<string, int>? GetOrBuild(IReadOnlyList<AbCipStructureMember> members)
+    {
+        ArgumentNullException.ThrowIfNull(members);
+        return Cache.GetValue(members, m => new LayoutBox(TryBuild(m))).Offsets;
+    }
+
     /// <summary>
     ///     Try to compute member offsets for the supplied declared members. Returns <c>null</c>
     ///     if any member type is unsupported for declaration-only layout.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip/AbCipUdtReadPlanner.cs

@@ -67,7 +67,7 @@ public static class AbCipUdtReadPlanner
                 continue;
             }
 
-            var offsets = AbCipUdtMemberLayout.TryBuild(parent.Members);
+            var offsets = AbCipUdtMemberLayout.GetOrBuild(parent.Members);
             if (offsets is null || !offsets.TryGetValue(memberName, out var offset))
             {
                 fallback.Add(new AbCipUdtReadFallback(i, name));

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests/AbCipUdtMemberLayoutTests.cs

@@ -99,4 +99,26 @@ public sealed class AbCipUdtMemberLayoutTests
             new AbCipStructureMember("Setpoints", AbCipDataType.Real, ElementCount: 4),
         }).ShouldBeNull();
     }
+
+    /// <summary>Review 07 P11: the layout is computed once per member list, not once per member per poll.</summary>
+    [Fact]
+    public void GetOrBuild_computes_a_layout_once_per_member_list()
+    {
+        IReadOnlyList<AbCipStructureMember> members =
+        [
+            new AbCipStructureMember("A", AbCipDataType.DInt),
+            new AbCipStructureMember("B", AbCipDataType.Real),
+        ];
+        var first = AbCipUdtMemberLayout.GetOrBuild(members);
+        first.ShouldNotBeNull();
+        first["B"].ShouldBe(4);
+        AbCipUdtMemberLayout.GetOrBuild(members).ShouldBeSameAs(first);
+
+        // Positive control: a different (equal-content) list is its own entry, and a not-groupable verdict caches too.
+        IReadOnlyList<AbCipStructureMember> copy = [.. members];
+        AbCipUdtMemberLayout.GetOrBuild(copy).ShouldNotBeSameAs(first);
+        IReadOnlyList<AbCipStructureMember> withBool = [new AbCipStructureMember("F", AbCipDataType.Bool)];
+        AbCipUdtMemberLayout.GetOrBuild(withBool).ShouldBeNull();
+        AbCipUdtMemberLayout.GetOrBuild(withBool).ShouldBeNull();
+    }
 }