test(host): serialize Central-boot fixtures to close env-var race

CentralDbTestEnvironment sets five process-wide environment variables, and
Program's AddEnvironmentVariables() reads them at an unpredictable point during
host boot. With xUnit collection parallelization on, one fixture's teardown
could clear a var mid-boot for a sibling. Since the secrets adoption (G-4)
three of those keys are ${secret:...} references that fail closed, turning a
previously benign empty value into a SecretNotFoundException that aborts the
boot — an intermittent CI failure.

Adds a HostBootCollection that serializes every fixture booting a real host
while depending on that shared state, folding in the narrower "ActorSystem"
collection so its members stay serialized with each other as before. Site-role
fixtures stay parallel: they call Configuration.Sources.Clear(), dropping the
env-var provider, so they cannot participate in the race.

CentralDbTestEnvironment now also fails fast if two instances are ever live at
once, making a regression (a fixture added outside the collection) deterministic
rather than intermittent — this is what surfaced the disposed-CTS defect fixed
in the previous commit. Fixture teardown is try/finally so a throwing host
teardown can no longer strand the vars for the rest of the run.

Cost: Host.Tests runs ~2m30s -> ~4m10s; the serialized Central-boot classes are
most of the assembly's parallelism.

Fixes: Gitea #15

tests/ZB.MOM.WW.ScadaBridge.Host.Tests/AkkaHostedServiceAuditWiringTests.cs

@@ -86,6 +86,7 @@ public class AkkaHostedServiceAuditWiringHoconTests
 /// <summary>
 /// Verifies Audit Log (#23) services land in the Central composition root.
 /// </summary>
+[Collection(HostBootCollection.Name)]
 public class CentralAuditWiringTests : IDisposable
 {
     private readonly WebApplicationFactory<Program> _factory;
@@ -156,9 +157,17 @@ public class CentralAuditWiringTests : IDisposable
 
     public void Dispose()
     {
-        _factory.Dispose();
-        Environment.SetEnvironmentVariable("DOTNET_ENVIRONMENT", _previousEnv);
-        _dbEnv.Dispose();
+        // try/finally: a throwing host teardown must not strand the process-wide
+        // environment variables for the rest of the run.
+        try
+        {
+            _factory.Dispose();
+        }
+        finally
+        {
+            Environment.SetEnvironmentVariable("DOTNET_ENVIRONMENT", _previousEnv);
+            _dbEnv.Dispose();
+        }
     }
 
     [Fact]