fix(worker): forget write-completion entries for dropped ack item handles

Add MxAccessWriteCompletionCache.Remove and RemoveServer and call them
from AlarmAckWriter when it drops an item (dirty re-add, LRU eviction,
failed AddItem/Advise/Write) and when it tears down its registration, so
the cache no longer keeps one entry per dropped handle for the life of
the worker.

Fixes #165

Claude-Session: https://claude.ai/code/session_016J6isfcgqy4SB2RK3ZvFvT

src/ZB.MOM.WW.MxGateway.Worker.Tests/MxAccess/AlarmAckWriterTests.cs

@@ -333,6 +333,76 @@ public sealed class AlarmAckWriterTests
         Assert.Empty(server.RemovedItemHandles);
     }
 
+    [Fact]
+    public void Acknowledge_AfterDirtyReAdd_ForgetsDroppedHandleCompletion()
+    {
+        RecordingAckServer server = new RecordingAckServer { AuthenticateResult = 5 };
+        MxAccessWriteCompletionCache cache = new MxAccessWriteCompletionCache();
+        using AlarmAckWriter writer = new AlarmAckWriter(server, cache, () => { }, TimeSpan.FromMilliseconds(20), "MxGateway");
+
+        writer.Acknowledge("T.A.AckMsg", "one", Alice()); // handle 100, Unconfirmed
+        cache.Record(7, 100, Rows(MxStatusCategory.Ok, 0)); // its late completion
+        Assert.Equal(1, cache.Count);
+        server.OnWrite = (handle, item) => cache.Record(handle, item, Rows(MxStatusCategory.Ok, 0));
+
+        AlarmAckResult second = writer.Acknowledge("T.A.AckMsg", "two", Alice()); // drops 100, adds 101
+
+        Assert.Equal(AlarmAckOutcome.Applied, second.Outcome);
+        Assert.Equal(1, cache.Count);
+        Assert.Equal(0UL, cache.CurrentVersion(7, 100));
+        Assert.Equal(1UL, cache.CurrentVersion(7, 101));
+    }
+
+    [Fact]
+    public void Acknowledge_PastCacheCap_ForgetsEvictedHandleCompletion()
+    {
+        RecordingAckServer server = new RecordingAckServer { AuthenticateResult = 5 };
+        MxAccessWriteCompletionCache cache = new MxAccessWriteCompletionCache();
+        server.OnWrite = (handle, item) => cache.Record(handle, item, Rows(MxStatusCategory.Ok, 0));
+        using AlarmAckWriter writer = new AlarmAckWriter(server, cache, () => { }, TimeSpan.FromSeconds(1), "MxGateway", maxCachedItems: 2);
+
+        writer.Acknowledge("A.X.AckMsg", "1", Alice()); // handle 100
+        writer.Acknowledge("B.X.AckMsg", "2", Alice()); // handle 101
+        Assert.Equal(2, cache.Count);
+        writer.Acknowledge("C.X.AckMsg", "3", Alice()); // handle 102, evicts A (100)
+
+        Assert.Equal(2, cache.Count);
+        Assert.Equal(0UL, cache.CurrentVersion(7, 100));
+    }
+
+    [Fact]
+    public void Dispose_ForgetsServerCompletions()
+    {
+        RecordingAckServer server = new RecordingAckServer { AuthenticateResult = 5 };
+        MxAccessWriteCompletionCache cache = new MxAccessWriteCompletionCache();
+        server.OnWrite = (handle, item) => cache.Record(handle, item, Rows(MxStatusCategory.Ok, 0));
+        AlarmAckWriter writer = new AlarmAckWriter(server, cache, () => { }, TimeSpan.FromSeconds(1), "MxGateway");
+        writer.Acknowledge("A.X.AckMsg", "1", Alice());
+        writer.Acknowledge("B.X.AckMsg", "2", Alice());
+        cache.Record(8, 100, Rows(MxStatusCategory.Ok, 0)); // another server's entry survives
+
+        writer.Dispose();
+
+        Assert.Equal(1, cache.Count);
+        Assert.Equal(1UL, cache.CurrentVersion(8, 100));
+    }
+
+    [Fact]
+    public void Acknowledge_WhenWriteDisconnected_ForgetsServerCompletions()
+    {
+        RecordingAckServer server = new RecordingAckServer { AuthenticateResult = 5 };
+        MxAccessWriteCompletionCache cache = new MxAccessWriteCompletionCache();
+        server.OnWrite = (handle, item) => cache.Record(handle, item, Rows(MxStatusCategory.Ok, 0));
+        using AlarmAckWriter writer = new AlarmAckWriter(server, cache, () => { }, TimeSpan.FromSeconds(1), "MxGateway");
+        writer.Acknowledge("A.X.AckMsg", "1", Alice()); // handle 100 recorded
+        server.WriteException = new COMException("x", unchecked((int)0x80010108));
+
+        writer.Acknowledge("A.X.AckMsg", "2", Alice());
+
+        Assert.Contains("Unregister", server.Operations);
+        Assert.Equal(0, cache.Count);
+    }
+
     [Fact]
     public void Acknowledge_WhenNonOkCompletionHasNoDetail_ReturnsMinusOne()
     {