fix(scripted-alarms): resolve Medium code-review finding (Core.ScriptedAlarms-002)

Dispose any existing _shelvingTimer before reassigning it inside LoadAsync so
that a second LoadAsync call does not leak the old timer and leave two timers
running concurrently against the same engine state.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Core/ZB.MOM.WW.OtOpcUa.Core.ScriptedAlarms/ScriptedAlarmEngine.cs

@@ -165,6 +165,11 @@ public sealed class ScriptedAlarmEngine : IDisposable
             _loaded = true;
             _engineLogger.Information("ScriptedAlarmEngine loaded {Count} alarm(s)", _alarms.Count);
 
+            // Dispose any previously-created timer before reassigning; a second LoadAsync
+            // call without this would leave two timers firing against the same engine.
+            // (Core.ScriptedAlarms-002)
+            _shelvingTimer?.Dispose();
+
             // Start the shelving-check timer — ticks every 5s, expires any timed shelves
             // that have passed their UnshelveAtUtc.
             _shelvingTimer = new Timer(_ => RunShelvingCheck(),