feat(lmxproxy): replace subscribe/unsubscribe health probe with persistent subscription

The old probe did a subscribe-read-unsubscribe cycle every 5 seconds to
check connection health. This created unnecessary churn and didn't detect
the failure mode where long-lived subscriptions silently stop receiving
COM callbacks (e.g. stalled STA message pump). The new approach keeps a
persistent subscription on the health check tag and forces reconnect if
no value update arrives within a configurable threshold (ProbeStaleThresholdMs,
default 5s). Also adds STA message pump debug logging (5-min heartbeat with
message counters) and fixes log file path resolution for Windows services.

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

lmxproxy/src/ZB.MOM.WW.LmxProxy.Host/Domain/IScadaClient.cs

@@ -57,12 +57,6 @@ namespace ZB.MOM.WW.LmxProxy.Host.Domain
             int pollIntervalMs,
             CancellationToken ct = default);
 
-        /// <summary>
-        ///     Probes connection health by reading a test tag.
-        ///     Returns a classified result: Healthy, TransportFailure, or DataDegraded.
-        /// </summary>
-        Task<ProbeResult> ProbeConnectionAsync(string testTagAddress, int timeoutMs, CancellationToken ct = default);
-
         /// <summary>
         ///     Unsubscribes specific tag addresses. Removes from stored subscriptions
         ///     and COM state. Safe to call after reconnect -- uses current handle mappings.