perf(historian): group-commit batch append on the historization outbox

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Tests/Recorder/FasterLogHistorizationOutboxTests.cs

@@ -140,6 +140,62 @@ public sealed class FasterLogHistorizationOutboxTests : IDisposable
         Assert.Equal(new[] { a.Id, b.Id }, batch.Select(e => e.Id));
     }
 
+    [Fact]
+    public async Task AppendBatch_then_peek_returns_fifo_in_batch_order()
+    {
+        // Group commit: one fsync for the whole batch. The reopen leg is the load-bearing half — it
+        // proves the single per-batch commit actually made every entry durable, not just the last one.
+        var dir = NewTempDir();
+        var a = E("A", 1);
+        var b = E("B", 2);
+        var c = E("C", 3);
+        {
+            using var o = new FasterLogHistorizationOutbox(dir, HistorizationCommitMode.PerEntry);
+            await o.AppendBatchAsync([a, b, c], TestContext.Current.CancellationToken);
+
+            Assert.Equal(3, await o.CountAsync(TestContext.Current.CancellationToken));
+            var batch = await o.PeekBatchAsync(10, TestContext.Current.CancellationToken);
+            Assert.Equal(new[] { a.Id, b.Id, c.Id }, batch.Select(e => e.Id));
+        }
+
+        using var reopened = new FasterLogHistorizationOutbox(dir, HistorizationCommitMode.PerEntry);
+        Assert.Equal(3, await reopened.CountAsync(TestContext.Current.CancellationToken));
+        var survived = await reopened.PeekBatchAsync(10, TestContext.Current.CancellationToken);
+        Assert.Equal(new[] { a.Id, b.Id, c.Id }, survived.Select(e => e.Id));
+    }
+
+    [Fact]
+    public async Task AppendBatch_at_capacity_drops_oldest_across_the_whole_batch()
+    {
+        // The drop-oldest loop runs ONCE for the batch, so an over-capacity batch must shed every
+        // surplus entry (not just one) and count each drop.
+        var dir = NewTempDir();
+        using var o = new FasterLogHistorizationOutbox(dir, HistorizationCommitMode.PerEntry, capacity: 2);
+        var a = E("A", 1);
+        var b = E("B", 2);
+        var c = E("C", 3);
+        var d = E("D", 4);
+        await o.AppendBatchAsync([a, b, c, d], TestContext.Current.CancellationToken);
+
+        Assert.Equal(2, await o.CountAsync(TestContext.Current.CancellationToken));
+        Assert.Equal(2, o.DroppedCount);
+        var tags = (await o.PeekBatchAsync(10, TestContext.Current.CancellationToken)).Select(e => e.Tag).ToArray();
+        Assert.Equal(new[] { "C", "D" }, tags);
+    }
+
+    [Fact]
+    public async Task AppendBatch_empty_is_noop()
+    {
+        var dir = NewTempDir();
+        using var o = new FasterLogHistorizationOutbox(dir, HistorizationCommitMode.PerEntry);
+        await o.AppendAsync(E("A", 1), TestContext.Current.CancellationToken);
+
+        await o.AppendBatchAsync([], TestContext.Current.CancellationToken);
+
+        Assert.Equal(1, await o.CountAsync(TestContext.Current.CancellationToken));
+        Assert.Equal(0, o.DroppedCount);
+    }
+
     [Fact]
     public async Task Remove_unknown_id_is_noop()
     {