feat(health): 0.2.0 — optional per-entry data + Akka cluster-view

Phase 0 of docs/plans/2026-07-22-overview-dashboard-impl-plan.md: give the
canonical health JSON a structured channel so the family overview dashboard can
read each Akka cluster's current leader.

- ZbHealthWriter: optional `"data": {...}` per entry, sourced from
  HealthReportEntry.Data, emitted only when non-empty. Per-property JsonIgnore
  (NOT a global DefaultIgnoreCondition) so `"description": null` still renders —
  payloads from data-less checks stay byte-identical to 0.1.0.
- AkkaClusterHealthCheck: BuildClusterData publishes this node's own view —
  leader (omitted while unknown), selfAddress, selfRoles (sorted), memberCount,
  unreachableCount — on every result path. The startup-safety paths (no
  ActorSystem / cluster inaccessible) stay description-only.
- Tests: writer data emit/omit (raw-JSON assert on the omit case), and a real
  single-node self-joined cluster via Akka.TestKit.Xunit2 for the data values.
  70 tests green (25/39/6).
- Version 0.1.0 -> 0.2.0; 3 packages published to the Gitea feed and
  restore-verified from a scratch consumer, which serves data.leader live.

ZB.MOM.WW.Health/tests/ZB.MOM.WW.Health.Akka.Tests/ZB.MOM.WW.Health.Akka.Tests.csproj

@@ -5,6 +5,7 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="Akka.TestKit.Xunit2" />
     <PackageReference Include="coverlet.collector" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" />
     <PackageReference Include="xunit" />