fix(scripted-alarms): resolve Medium code-review finding (Core.ScriptedAlarms-005)

Add _disposed re-checks inside ReevaluateAsync and ShelvingCheckAsync after
acquiring _evalGate so callbacks in flight when Dispose() runs bail out cleanly
instead of mutating _alarms or writing to a disposed store. Drop the
_alarms.Clear() from Dispose() — clearing outside the gate races concurrent
reads and is unnecessary since the object is being discarded.

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

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

@@ -266,6 +266,12 @@ public sealed class ScriptedAlarmEngine : IDisposable
             await _evalGate.WaitAsync(ct).ConfigureAwait(false);
             try
             {
+                // Re-check after acquiring the gate: a Dispose() call may have
+                // completed between our _evalGate.WaitAsync and here. Writing to a
+                // disposing store or mutating _alarms after clear is unsafe.
+                // (Core.ScriptedAlarms-005)
+                if (_disposed) return;
+
                 foreach (var id in alarmIds)
                 {
                     if (!_alarms.TryGetValue(id, out var state)) continue;
@@ -402,6 +408,13 @@ public sealed class ScriptedAlarmEngine : IDisposable
             await _evalGate.WaitAsync(ct).ConfigureAwait(false);
             try
             {
+                // Re-check after acquiring the gate: Timer.Dispose() does not wait for
+                // running callbacks, so a shelving-check callback that passed the _disposed
+                // check in RunShelvingCheck can arrive here after Dispose() has returned.
+                // Mutating _alarms or saving to a disposed store here is unsafe.
+                // (Core.ScriptedAlarms-005)
+                if (_disposed) return;
+
                 var now = _clock();
                 foreach (var id in alarmIds)
                 {
@@ -445,7 +458,11 @@ public sealed class ScriptedAlarmEngine : IDisposable
         _disposed = true;
         _shelvingTimer?.Dispose();
         UnsubscribeFromUpstream();
-        _alarms.Clear();
+        // Do NOT clear _alarms here: Timer.Dispose() does not wait for in-flight callbacks,
+        // so a ShelvingCheckAsync or ReevaluateAsync can still be running inside _evalGate.
+        // Those paths now re-check _disposed after acquiring the gate and bail out safely.
+        // Clearing _alarms outside the gate would race concurrent reads and is unnecessary
+        // (the whole object is being discarded). (Core.ScriptedAlarms-005)
         _alarmsReferencing.Clear();
     }