Three fixes for the SubscriptionManager/MxAccessClient subscription pipeline:
1. Serialize Subscribe and UnsubscribeClient with a SemaphoreSlim gate to prevent
race where old-session unsubscribe removes new-session COM subscriptions.
CreateMxAccessSubscriptionsAsync is now awaited instead of fire-and-forget.
2. Fix dual VTQ delivery in MxAccessClient.OnDataChange — each update was delivered
twice (once via stored callback, once via OnTagValueChanged property). Now uses
stored callback as the single delivery path.
3. Store pending tag addresses when CreateMxAccessSubscriptionsAsync fails (MxAccess
down) and retry them on reconnect via NotifyReconnection/RetryPendingSubscriptionsAsync.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
