fix(historian): volatile _backoffIndex + read _evictedCount under lock (thread-safety)

src/Core/ZB.MOM.WW.OtOpcUa.Core.AlarmHistorian/SqliteStoreAndForwardSink.cs

@@ -71,7 +71,7 @@ public sealed class SqliteStoreAndForwardSink : IAlarmHistorianSink, IDisposable
     private readonly SemaphoreSlim _drainGate = new(1, 1);
     private Timer? _drainTimer;
     private TimeSpan _tickInterval;
-    private int _backoffIndex;
+    private volatile int _backoffIndex;
     private bool _disposed;
 
     // Core.AlarmHistorian-005: status fields written by the drain timer thread and
@@ -655,10 +655,11 @@ public sealed class SqliteStoreAndForwardSink : IAlarmHistorianSink, IDisposable
             await cmd.ExecuteNonQueryAsync(ct).ConfigureAwait(false);
         }
         Interlocked.Add(ref _queuedRowCount, -toEvict);
-        lock (_statusLock) { _evictedCount += toEvict; }
+        long lifetimeEvicted;
+        lock (_statusLock) { _evictedCount += toEvict; lifetimeEvicted = _evictedCount; }
         _logger.Warning(
             "Historian queue at capacity {Cap} — evicted {Count} oldest row(s) to make room (lifetime evictions: {Total})",
-            _capacity, toEvict, _evictedCount);
+            _capacity, toEvict, lifetimeEvicted);
     }
 
     private void PurgeAgedDeadLetters(SqliteConnection conn)