test(harness): serialize Host.IntegrationTests; drop the reconciler's unused singleton proxy

Closes the last branch-only failures. Measured across five full-suite runs on
this branch versus two on master in a clean worktree: master failed only AbCip
in this assembly (2/2), while the branch additionally failed one or two E2E
deploy tests, rotating between DeployHappyPath, DriverReconnect and
EquipmentNamespaceMaterialization (4/4). Each passed in isolation, and the
assembly on its own was 195/201 throughout — the failures only appeared under
full-suite CPU contention.

Cause is contention, not correctness. Every test here builds a real two-node
Akka cluster — two Kestrel hosts, two ActorSystems with remoting, six admin
singletons, an EF context, a deploy pipeline — and xUnit ran them concurrently
with each other and with every other assembly. That was survivable while the
coordinator sealed instantly on an empty expected-ack set; now that it waits for
a real ack from every configured node, these tests measure an end-to-end
round-trip and starve.

Serialising this one assembly makes them deterministic: 195/201 with only
AbCip_Green_AgainstSim, and the full-solution failure set is now IDENTICAL to
master's — 7 shared, zero branch-only. Cost is wall-clock for this assembly,
40s -> 3m35s; it is a handful of heavyweight E2E tests, not a broad unit suite,
and a widened timeout alone did not fix it (tried first, at 45s).

Also drops createProxyToo for the address reconciler. Nothing resolves
ClusterNodeAddressReconcilerKey from the registry — the actor only reads cluster
state and logs — so the proxy was a second actor per node hunting for a
singleton nobody addresses. Kept because it removes dead machinery, not because
it fixed the flakiness; measured on its own, it did not.

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

src/Server/ZB.MOM.WW.OtOpcUa.ControlPlane/ServiceCollectionExtensions.cs

@@ -26,7 +26,8 @@ public static class ServiceCollectionExtensions
     public const string ClusterNodeAddressReconcilerSingletonName = "cluster-node-address-reconciler";
 
     /// <summary>
-    /// Registers all five admin-role cluster singletons + their proxies on the AkkaConfigurationBuilder.
+    /// Registers the admin-role cluster singletons on the AkkaConfigurationBuilder — five with
+    /// registry proxies, plus the address reconciler, which is proxy-less because nothing addresses it.
     /// Must be called against the same builder used by <c>AkkaHostedService</c> so the singletons
     /// share the host's ActorSystem.
     /// </summary>
@@ -94,6 +95,13 @@ public static class ServiceCollectionExtensions
         // Per-cluster mesh Phase 1: ClusterNode.AkkaPort duplicates the node's own Cluster:Port and
         // nothing makes them agree. Runs here rather than on each driver node because Phase 4 removes
         // the driver nodes' ConfigDb connection entirely.
+        //
+        // createProxyToo: false — unlike the five singletons above, nothing ever resolves
+        // ClusterNodeAddressReconcilerKey from the registry. This actor only reads cluster state and
+        // logs; no one sends it messages. A proxy would be a second actor per node whose only job is
+        // to hunt for a singleton nobody addresses — visible as the "ClusterSingletonProxy failed to
+        // find an associated singleton after 30 seconds" startup warning, and as extra cluster
+        // chatter in every two-node test harness the integration suite builds.
         builder.WithSingleton<ClusterNodeAddressReconcilerKey>(
             ClusterNodeAddressReconcilerSingletonName,
             (system, registry, resolver) =>
@@ -101,7 +109,8 @@ public static class ServiceCollectionExtensions
                 var dbFactory = resolver.GetService<IDbContextFactory<OtOpcUaConfigDbContext>>();
                 return ClusterNodeAddressReconcilerActor.Props(dbFactory);
             },
-            singletonOptions);
+            singletonOptions,
+            createProxyToo: false);
 
         return builder;
     }

tests/Server/ZB.MOM.WW.OtOpcUa.Host.IntegrationTests/AssemblyInfo.cs

@@ -0,0 +1,14 @@
+using Xunit;
+
+// Every test in this assembly builds a real two-node Akka cluster: two in-process Kestrel hosts,
+// two ActorSystems with remoting on ephemeral ports, six admin-role cluster singletons, an EF
+// context and a deploy pipeline. Running several of those concurrently — on top of the other test
+// assemblies dotnet test runs in parallel — starves the cluster-formation and ApplyAck timers these
+// tests measure, so failures appear under full-suite load and vanish in isolation.
+//
+// That was tolerable while the deploy path sealed immediately on an empty expected-ack set. Since
+// per-cluster mesh Phase 1 the coordinator waits for a real ack from every configured node, so these
+// tests now measure an end-to-end round-trip and are correspondingly more sensitive to CPU
+// starvation. Serialising this assembly trades wall-clock for determinism; the assembly is a handful
+// of heavyweight E2E tests, not a broad unit suite.
+[assembly: CollectionBehavior(DisableTestParallelization = true)]