feat(ablegacy): implement IBatchSubscribable

Review 02 P1 / Wave 2 rule (d): AbLegacy runs PollGroupEngine but raised only
the per-tag OnDataChange, so every changed tag paid its own trip through the
actor chain. The engine's per-cycle batch is now raised as OnDataChangeBatch
(one per poll cycle), alongside the per-tag event for legacy consumers.

Refs #738
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs

@@ -12,7 +12,7 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.AbLegacy;
 ///     <see cref="IDriver"/> only at PR 1 time; read / write / discovery / subscribe / probe /
 ///     host-resolver capabilities ship in PRs 2 and 3.
 /// </summary>
-public sealed partial class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscovery, ISubscribable,
+public sealed partial class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscovery, IBatchSubscribable,
     IHostConnectivityProbe, IPerCallHostResolver, ISkippedTagReporter, IDisposable, IAsyncDisposable
 {
     /// <summary>Authored tags this driver could not build into its poll table.</summary>
@@ -64,6 +64,12 @@ public sealed partial class AbLegacyDriver : IDriver, IReadable, IWritable, ITag
     /// </summary>
     public event EventHandler<DataChangeEventArgs>? OnDataChange;
 
+    /// <summary>
+    ///     One raise per poll cycle carrying every tag that changed in it (review 02 P1). Raised alongside the per-tag
+    ///     <see cref="OnDataChange"/>; a consumer subscribes to exactly one of the two.
+    /// </summary>
+    public event EventHandler<DataChangeBatchEventArgs>? OnDataChangeBatch;
+
     /// <summary>
     ///     Occurs when host status changes.
     /// </summary>
@@ -99,6 +105,8 @@ public sealed partial class AbLegacyDriver : IDriver, IReadable, IWritable, ITag
                 OnDataChange?.Invoke(this, new DataChangeEventArgs(handle, tagRef, snapshot)),
             onError: HandlePollError,
             backoffCap: PollBackoffCap,
+            onBatch: (handle, items) =>
+                OnDataChangeBatch?.Invoke(this, new DataChangeBatchEventArgs(handle, items)),
             // The engine owns no Meter (Core.Abstractions takes no metrics dependency), so this
             // closure IS the production consumer of its overrun signal.
             onOverrun: _ => OtOpcUaTelemetry.PollOverruns.Add(1,