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,

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyBatchSubscribeTests.cs

@@ -0,0 +1,50 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests;
+
+/// <summary>
+///     Review 02 P1: AbLegacy ran <see cref="PollGroupEngine"/> but did not implement <see cref="IBatchSubscribable"/>,
+///     so every changed tag paid its own trip through the actor chain. It now raises one
+///     <see cref="IBatchSubscribable.OnDataChangeBatch"/> per poll cycle, alongside the per-tag event.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class AbLegacyBatchSubscribeTests
+{
+    private const string Host = "ab://10.0.0.5/1,0";
+
+    [Fact]
+    public async Task One_batch_per_poll_carries_every_changed_tag_and_per_tag_events_still_fire()
+    {
+        var factory = new FakeAbLegacyTagFactory { Customise = p => new FakeAbLegacyTag(p) { Value = p.TagName == "N7:0" ? 1 : 2 } };
+        var drv = new AbLegacyDriver(new AbLegacyDriverOptions
+        {
+            Devices = [new AbLegacyDeviceOptions(Host)],
+            RawTags = AbLegacyRawTags.Entries(
+                new AbLegacyTagDefinition("A", Host, "N7:0", AbLegacyDataType.Int),
+                new AbLegacyTagDefinition("B", Host, "N9:0", AbLegacyDataType.Int)),
+            Probe = new AbLegacyProbeOptions { Enabled = false },
+        }, "drv-batch", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        IBatchSubscribable batching = drv; // the capability the actor prefers
+
+        var batches = new System.Collections.Concurrent.ConcurrentQueue<DataChangeBatchEventArgs>();
+        var singles = new System.Collections.Concurrent.ConcurrentQueue<DataChangeEventArgs>();
+        batching.OnDataChangeBatch += (_, e) => batches.Enqueue(e);
+        drv.OnDataChange += (_, e) => singles.Enqueue(e);
+
+        var handle = await drv.SubscribeAsync(["A", "B"], TimeSpan.FromMilliseconds(100), CancellationToken.None);
+        var deadline = DateTime.UtcNow.AddSeconds(5);
+        while (batches.IsEmpty && DateTime.UtcNow < deadline) await Task.Delay(10);
+
+        batches.TryPeek(out var first).ShouldBeTrue();
+        first!.SubscriptionHandle.ShouldBe(handle);
+        first.Items.Select(i => i.FullReference).ShouldBe(new[] { "A", "B" }, ignoreOrder: true);
+        first.Items.Single(i => i.FullReference == "A").Snapshot.Value.ShouldBe(1);
+        singles.Count.ShouldBeGreaterThanOrEqualTo(2); // positive control: the legacy per-tag event still fires
+
+        await drv.UnsubscribeAsync(handle, CancellationToken.None);
+        await drv.ShutdownAsync(CancellationToken.None);
+    }
+}