fix(health): active tier returns 503 on a role-member-but-not-leader node (0.2.1)

The shared spec states the active tier "Fails (503) on a standby or
role-member-but-not-leader node", and the same spec maps Degraded to HTTP 200.
ActiveNodeDecision.Evaluate returned Degraded for exactly that case, so the tier
answered 200 on every node and could not distinguish an active node from a
standby over HTTP at all - only the response body differed.

The consequence was live, not theoretical: OtOpcUa's Traefik routes the admin UI
by this probe (docker-dev/traefik-dynamic.yml, scripts/install/traefik-dynamic.yml),
so both central nodes always passed and the leader-pinning the design called for
has silently never worked. Found while running the overview dashboard's live
acceptance, which reads the same tier and showed all six rig nodes as Active.
Filed as lmxopcua#494.

Role-member-but-not-leader now returns Unhealthy. The other two branches are
unchanged: a node that lacks the role stays Healthy (the probe is irrelevant to
it), and the startup-safety path still returns Degraded when the ActorSystem or
cluster is not yet available.

Blast radius is one consumer. OtOpcUa is the only user of the role-filtered mode;
ScadaBridge deliberately uses its own OldestNodeActiveHealthCheck (leadership is
address-ordered and diverges from the singleton host), and MxGateway and
HistorianGateway have no Akka cluster.

Version 0.2.1, three packages published to the Gitea feed and restore-verified.
70 tests pass.

ZB.MOM.WW.Health/Directory.Build.props

@@ -5,7 +5,7 @@
     <Nullable>enable</Nullable>
     <ImplicitUsings>enable</ImplicitUsings>
     <LangVersion>latest</LangVersion>
-    <Version>0.2.0</Version>
+    <Version>0.2.1</Version>
     <ManagePackageVersionsCentrally>true</ManagePackageVersionsCentrally>
     <!-- Emit XML docs so the public API summaries ship inside the packed nupkgs (IntelliSense for
          consumers). CS1591 (missing doc on a public member) is suppressed so undocumented test /