Resolve 6 of 7 stability review findings and close test coverage gaps

Fixes P1 StaComThread hang (crash-path faulting via WorkItem queue), P1 subscription
fire-and-forget (block+log or ContinueWith on 5 call sites), P2 continuation point
leak (PurgeExpired on Retrieve/Release), P2 dashboard bind failure (localhost prefix,
bool Start), P3 background loop double-start (task handles + join on stop in 3 files),
and P3 config logging exposure (SqlConnectionStringBuilder password masking). Adds
FakeMxAccessClient fault injection and 12 new tests. Documents required runtime
assemblies in ServiceHosting.md.

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

src/ZB.MOM.WW.LmxOpcUa.Host/GalaxyRepository/ChangeDetectionService.cs

@@ -16,6 +16,7 @@ namespace ZB.MOM.WW.LmxOpcUa.Host.GalaxyRepository
 
         private readonly IGalaxyRepository _repository;
         private CancellationTokenSource? _cts;
+        private Task? _pollTask;
 
         /// <summary>
         ///     Initializes a new change detector for Galaxy deploy timestamps.
@@ -55,8 +56,11 @@ namespace ZB.MOM.WW.LmxOpcUa.Host.GalaxyRepository
         /// </summary>
         public void Start()
         {
+            if (_cts != null)
+                Stop();
+
             _cts = new CancellationTokenSource();
-            Task.Run(() => PollLoopAsync(_cts.Token));
+            _pollTask = Task.Run(() => PollLoopAsync(_cts.Token));
             Log.Information("Change detection started (interval={Interval}s)", _intervalSeconds);
         }
 
@@ -66,6 +70,8 @@ namespace ZB.MOM.WW.LmxOpcUa.Host.GalaxyRepository
         public void Stop()
         {
             _cts?.Cancel();
+            try { _pollTask?.Wait(TimeSpan.FromSeconds(5)); } catch { /* timeout or faulted */ }
+            _pollTask = null;
             Log.Information("Change detection stopped");
         }