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>

tests/ZB.MOM.WW.LmxOpcUa.Tests/Status/StatusWebServerTests.cs

@@ -145,7 +145,7 @@ namespace ZB.MOM.WW.LmxOpcUa.Tests.Status
             var server2 = new StatusWebServer(
                 new StatusReportService(new HealthCheckService(), 10),
                 new Random().Next(19000, 20000));
-            server2.Start();
+            server2.Start().ShouldBe(true);
             server2.IsRunning.ShouldBe(true);
             server2.Stop();
         }