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/MxAccess/AlarmAckWriter.cs

@@ -303,6 +303,9 @@ public sealed class AlarmAckWriter : IAlarmAckWriter
             try { mx.Unregister(serverHandle); } catch { /* best effort */ }
         }
 
+        // The registration is gone, so no completion can arrive for its handles.
+        completionCache.RemoveServer(serverHandle);
+
         itemHandles.Clear();
         itemLru.Clear();
         server = null;
@@ -335,6 +338,10 @@ public sealed class AlarmAckWriter : IAlarmAckWriter
         itemLru.Remove(node);
         try { mx.UnAdvise(serverHandle, node.Value.Handle); } catch { /* best effort */ }
         try { mx.RemoveItem(serverHandle, node.Value.Handle); } catch { /* best effort */ }
+
+        // Without this the completion cache keeps one entry per dropped handle
+        // (each Unconfirmed re-add and each eviction) for the worker's life.
+        completionCache.Remove(serverHandle, node.Value.Handle);
     }
 
     private IMxAccessServer EnsureRegistered()

src/ZB.MOM.WW.MxGateway.Worker/MxAccess/MxAccessWriteCompletionCache.cs

@@ -184,6 +184,57 @@ public sealed class MxAccessWriteCompletionCache
         }
     }
 
+    /// <summary>
+    ///     Forgets the completion entry of a handle pair whose MXAccess item was
+    ///     removed, so a long-lived writer that drops and re-adds items does not
+    ///     keep one entry per dropped handle for the life of the worker.
+    /// </summary>
+    /// <param name="serverHandle">MXAccess server handle.</param>
+    /// <param name="itemHandle">MXAccess item handle that was removed.</param>
+    public void Remove(
+        int serverHandle,
+        int itemHandle)
+    {
+        lock (syncRoot)
+        {
+            entries.Remove(CreateItemKey(serverHandle, itemHandle));
+        }
+    }
+
+    /// <summary>Forgets every completion entry recorded under a server handle that was unregistered.</summary>
+    /// <param name="serverHandle">MXAccess server handle that was unregistered.</param>
+    public void RemoveServer(int serverHandle)
+    {
+        lock (syncRoot)
+        {
+            List<long> stale = new();
+            foreach (long key in entries.Keys)
+            {
+                if ((int)(key >> 32) == serverHandle)
+                {
+                    stale.Add(key);
+                }
+            }
+
+            foreach (long key in stale)
+            {
+                entries.Remove(key);
+            }
+        }
+    }
+
+    /// <summary>Gets the number of handle pairs with a recorded completion (test seam).</summary>
+    internal int Count
+    {
+        get
+        {
+            lock (syncRoot)
+            {
+                return entries.Count;
+            }
+        }
+    }
+
     private static long CreateItemKey(
         int serverHandle,
         int itemHandle)