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.Tests/ResponseWriterTests.cs

@@ -22,8 +22,11 @@ public sealed class ResponseWriterTests
     {
         private readonly HealthCheckResult _result;
 
-        public StubHealthCheck(HealthStatus status, string? description = null) =>
-            _result = new HealthCheckResult(status, description);
+        public StubHealthCheck(
+            HealthStatus status,
+            string? description = null,
+            IReadOnlyDictionary<string, object>? data = null) =>
+            _result = new HealthCheckResult(status, description, exception: null, data: data);
 
         public Task<HealthCheckResult> CheckHealthAsync(
             HealthCheckContext context,
@@ -31,12 +34,14 @@ public sealed class ResponseWriterTests
     }
 
     private static async Task<HttpResponseMessage> GetReadyAsync(
-        HealthStatus status, string? description = "db reachable")
+        HealthStatus status,
+        string? description = "db reachable",
+        IReadOnlyDictionary<string, object>? data = null)
     {
         var builder = WebApplication.CreateBuilder();
         builder.WebHost.UseTestServer();
         builder.Services.AddHealthChecks()
-            .AddCheck("db", new StubHealthCheck(status, description), tags: new[] { ZbHealthTags.Ready });
+            .AddCheck("db", new StubHealthCheck(status, description, data), tags: new[] { ZbHealthTags.Ready });
 
         await using var app = builder.Build();
         app.MapZbHealth();
@@ -84,6 +89,51 @@ public sealed class ResponseWriterTests
         Assert.Equal(JsonValueKind.Null, description.ValueKind);
     }
 
+    [Fact]
+    public async Task ReadyEndpoint_EntryWithData_EmitsDataObject_WithVerbatimKeys()
+    {
+        // The envelope properties are camelCased by the naming policy, but a check's own data keys are
+        // dictionary keys and must survive verbatim — the dashboard reads them by exact name
+        // (entries["akka-cluster"].data.leader), so a naming policy applied here would silently break
+        // the contract. Values may be any JSON-friendly type (string / int / string[]).
+        var data = new Dictionary<string, object>
+        {
+            ["leader"] = "akka.tcp://sys@node-1:4053",
+            ["memberCount"] = 2,
+            ["selfRoles"] = new[] { "driver", "site-a" },
+        };
+
+        var response = await GetReadyAsync(HealthStatus.Healthy, data: data);
+
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+
+        using var doc = JsonDocument.Parse(await response.Content.ReadAsStringAsync());
+        var entryData = doc.RootElement.GetProperty("entries").GetProperty("db").GetProperty("data");
+
+        Assert.Equal("akka.tcp://sys@node-1:4053", entryData.GetProperty("leader").GetString());
+        Assert.Equal(2, entryData.GetProperty("memberCount").GetInt32());
+        Assert.Equal(
+            new[] { "driver", "site-a" },
+            entryData.GetProperty("selfRoles").EnumerateArray().Select(static e => e.GetString()).ToArray());
+    }
+
+    [Fact]
+    public async Task ReadyEndpoint_EntryWithoutData_OmitsDataKeyEntirely()
+    {
+        // The 0.2.0 non-breaking invariant: a check that publishes no data must produce exactly the
+        // pre-0.2.0 body. Asserted on the raw JSON string so a `"data": null` regression (which
+        // TryGetProperty would still accept as "present") fails here.
+        var response = await GetReadyAsync(HealthStatus.Healthy);
+
+        var body = await response.Content.ReadAsStringAsync();
+
+        Assert.DoesNotContain("\"data\"", body, StringComparison.Ordinal);
+
+        using var doc = JsonDocument.Parse(body);
+        var db = doc.RootElement.GetProperty("entries").GetProperty("db");
+        Assert.False(db.TryGetProperty("data", out _), "data key must be absent when the check set none");
+    }
+
     [Fact]
     public async Task ReadyEndpoint_Degraded_Returns200_WithDegradedStatus()
     {