refactor(health): adopt the shared active-node check (Health 0.3.0)

OldestNodeActiveHealthCheck existed because the shared ActiveNodeHealthCheck
selected by cluster leadership (review 01 [High]): leadership is address-ordered
and diverges from singleton placement after a restart, and during a partition
both sides compute themselves leader so Traefik served both. Health 0.3.0 makes
the shared check age-based — it is now this host's own rule, promoted — so the
private copy is deleted and central registers the shared type.

ActiveNodeEvaluator, the "THE single definition of active node" this repo already
maintained, now delegates to the shared ClusterActiveNode rather than
re-implementing it. That keeps the delivery gate, the heartbeat IsActive stamp,
the inbound-API gate and the /health/active tier on one rule, and it means the
rule is shared with OtOpcUa, which had independently written a third copy.
Communication takes a ZB.MOM.WW.Health.Akka reference for it; the layering
trade-off is recorded at the PackageReference.

SitePairActiveNodeHealthCheck is deliberately KEPT. It is not a duplicate of the
rule — it is a thin adapter over the site's own IClusterNodeProvider, which
scopes to site-{SiteId} and is itself now backed by ClusterActiveNode. Registering
the shared check directly would have to re-derive the site role and would lose the
property that the tier and singleton placement come from the same provider.
Central stays unscoped: every central member competes for one active slot.

No behaviour change on any node — same rule, one implementation instead of two.

Verified: Host.Tests 439/439, Communication ActiveNode 2/2.

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

@@ -5,6 +5,7 @@ using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Diagnostics.HealthChecks;
 using Microsoft.Extensions.Options;
 using ZB.MOM.WW.Health;
+using ZB.MOM.WW.Health.Akka;
 using ZB.MOM.WW.ScadaBridge.Host.Health;
 
 namespace ZB.MOM.WW.ScadaBridge.Host.Tests;
@@ -207,11 +208,13 @@ public class HealthCheckTests : IDisposable
     }
 
     [Fact]
-    public void ActiveNodeCheck_IsHostLocalOldestNodeCheck()
+    public void ActiveNodeCheck_IsTheSharedOldestUpMemberCheck()
     {
-        // The shared package's ActiveNodeHealthCheck is LEADER-based (review 01 [High]):
-        // during a partition both nodes think they lead and Traefik serves both. The
-        // active-node check must be the host-local oldest-member check instead.
+        // The property under test is that the active tier selects by AGE, not by leadership: during a
+        // partition both sides think they lead and Traefik would serve both (review 01 [High]).
+        // Through ZB.MOM.WW.Health 0.2.1 the shared check was leader-based, so this host carried a
+        // private OldestNodeActiveHealthCheck; 0.3.0 made the shared check age-based, so the private
+        // copy is gone and the shared type is now the correct answer here.
         var previousEnv = Environment.GetEnvironmentVariable("DOTNET_ENVIRONMENT");
         try
         {
@@ -222,7 +225,7 @@ public class HealthCheckTests : IDisposable
             Assert.Contains(ZbHealthTags.Active, active.Tags);
 
             var check = active.Factory(factory.Services);
-            Assert.IsType<OldestNodeActiveHealthCheck>(check);
+            Assert.IsType<ActiveNodeHealthCheck>(check);
         }
         finally
         {