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/SiteHealthCheckTests.cs

@@ -122,14 +122,18 @@ public class SiteHealthCheckTests : IDisposable
     }
 
     [Fact]
-    public void Site_ActiveNodeCheck_IsNotCentralsRoleLessOldestCheck()
+    public void Site_ActiveNodeCheck_IsRoleScoped_NotTheUnscopedCentralCheck()
     {
-        // Central's OldestNodeActiveHealthCheck calls SelfIsOldest(cluster) with no role argument.
-        // On a mesh carrying more than one site's members that computes "oldest" across the wrong
-        // member set, so a site node could report Active purely for being the oldest member overall.
+        // Central registers the shared ActiveNodeHealthCheck UNSCOPED — oldest Up member of the whole
+        // cluster. On a mesh carrying more than one site's members that computes "oldest" across the
+        // wrong member set, so a site node could report Active purely for being the oldest member
+        // overall. A site must answer for its own site-{SiteId} role, which SitePairActiveNodeHealthCheck
+        // does by delegating to the site's IClusterNodeProvider (itself now backed by the shared
+        // ClusterActiveNode, so the rule is shared even though the scoping is local).
         var check = Assert.Single(Registrations, r => r.Name == "active-node").Factory(_host.Services);
 
-        Assert.IsNotType<OldestNodeActiveHealthCheck>(check);
+        Assert.IsType<SitePairActiveNodeHealthCheck>(check);
+        Assert.IsNotType<ActiveNodeHealthCheck>(check);
     }
 
     [Theory]