Add alarm acknowledge plan and incorporate code review fixes

Adds alarm_ack.md documenting the two-way acknowledge flow (OPC UA client
writes AckMsg, Galaxy confirms via Acked data change). Includes external
code review fixes for subscriptions and node manager, and removes stale
plan files now superseded by component documentation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/ZB.MOM.WW.LmxOpcUa.Tests/Helpers/OpcUaServerFixtureTests.cs

@@ -85,6 +85,21 @@ namespace ZB.MOM.WW.LmxOpcUa.Tests.Helpers
             sw.Elapsed.TotalSeconds.ShouldBeLessThan(30);
         }
 
+        /// <summary>
+        /// Confirms that runtime callbacks arriving after shutdown are ignored cleanly.
+        /// </summary>
+        [Fact]
+        public async Task Stop_UnhooksNodeManagerFromMxAccessCallbacks()
+        {
+            var mxClient = new FakeMxAccessClient();
+            var fixture = OpcUaServerFixture.WithFakeMxAccessClient(mxClient);
+            await fixture.InitializeAsync();
+
+            await fixture.DisposeAsync();
+
+            Should.NotThrow(() => mxClient.SimulateDataChange("TestMachine_001.MachineID", Vtq.Good(42)));
+        }
+
         /// <summary>
         /// Confirms that the fake-backed fixture builds the seeded address space and Galaxy statistics.
         /// </summary>