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/Program.cs

@@ -17,7 +17,10 @@ namespace ZB.MOM.WW.LmxProxy.Host
                 .AddEnvironmentVariables()
                 .Build();
 
-            // 2. Configure Serilog
+            // 2. Set working directory to exe location so relative log paths resolve correctly
+            Environment.CurrentDirectory = AppDomain.CurrentDomain.BaseDirectory;
+
+            // 3. Configure Serilog
             Log.Logger = new LoggerConfiguration()
                 .ReadFrom.Configuration(configuration)
                 .Enrich.FromLogContext()
@@ -27,11 +30,11 @@ namespace ZB.MOM.WW.LmxProxy.Host
 
             try
             {
-                // 3. Bind configuration
+                // 4. Bind configuration
                 var config = new LmxProxyConfiguration();
                 configuration.Bind(config);
 
-                // 4. Configure Topshelf
+                // 5. Configure Topshelf
                 var exitCode = HostFactory.Run(host =>
                 {
                     host.UseSerilog();