feat(overview): scaffold the family dashboard + registry + health client

Phase 3 tasks 3.1-3.3 of docs/plans/2026-07-22-overview-dashboard-impl-plan.md.

Scaffold (3.1): a plain directory in scadaproj (NOT a nested git repo), slnx over
src/ + tests/, HistorianGateway-pattern Directory.Build.props (warnings-as-errors)
and nuget.config (nuget.org * + the Gitea feed scoped to ZB.MOM.WW.*). Inline
package pins, no CPM — the app convention here. NO Auth/Audit/Secrets packages:
the dashboard is anonymous and read-only by requirement.

Registry (3.2): OverviewOptions/ApplicationEntry/InstanceEntry bound from the
"Overview" section, plus EffectiveTimings resolving instance > application >
global overrides in ONE place so no caller re-implements the precedence.
OverviewOptionsValidator (OptionsValidatorBase + AddValidatedOptions) rejects an
empty registry, applications with no instances, duplicate names, non-absolute or
non-http(s) URLs, and non-positive intervals — and checks stale > poll on the
EFFECTIVE values, since an override at either level can invert that on one
instance while the globals look fine. A ConfigPreflight presence check runs before
the host is built so a missing registry fails with the key name, not with
"Applications must have at least 1 entry".

Health client (3.3): ZbHealthReport DTOs for the canonical ZbHealthWriter body,
with per-entry `data` kept as JsonElement — it is an open contract, and binding it
to concrete types would break the moment a check adds a field. 200 AND 503 are
both parseable answers (503 carries the body naming the failing check); only a
transport failure or a non-canonical body is a failed probe, which is why
Unreachable stays distinct from Down. Host shutdown propagates as cancellation
rather than being recorded as every instance timing out.

Status model: Up/Degraded/Down/Unreachable + Active/Standby/Unknown/NotApplicable,
with two-strike flap damping — a failing observation only replaces a good state
after 2 consecutive failures, while recovery is immediate (damping suppresses
false alarms; symmetric damping would just make them linger).

53 tests green, 0 warnings in Debug and Release. Tests for the validator, the
client golden payloads (with AND without `data`, so a partly-bumped fleet renders)
and the derivation table are front-loaded here from task 3.8 so this batch is
verified rather than pending.

Also pins AngleSharp 1.5.2 test-side: bunit 1.40.0 pulls 1.2.0, which trips NU1902
and therefore the warnings-as-errors gate (same fix HistorianGateway made in
6bc005d).

ZB.MOM.WW.Overview/tests/ZB.MOM.WW.Overview.Tests/ZB.MOM.WW.Overview.Tests.csproj

@@ -0,0 +1,33 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <!--
+    Test stack mirrors ZB.MOM.WW.Theme's (xunit 2.9.3 + bunit), so component tests here read the
+    same way as the kit's own.
+  -->
+  <PropertyGroup>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
+    <PackageReference Include="xunit" Version="2.9.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.4" />
+    <PackageReference Include="bunit" Version="1.40.0" />
+    <!-- Transitive-security pin, test-only: bunit 1.40.0 pulls AngleSharp 1.2.0, which carries
+         NU1902 (GHSA-pgww-w46g-26qg) and therefore breaks the warnings-as-errors gate. Pinning the
+         patched version here is the same fix HistorianGateway applied (6bc005d); AngleSharp never
+         reaches the shipped app, only the bUnit renderer. -->
+    <PackageReference Include="AngleSharp" Version="1.5.2" />
+    <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="10.0.7" />
+    <PackageReference Include="coverlet.collector" Version="6.0.4" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\ZB.MOM.WW.Overview\ZB.MOM.WW.Overview.csproj" />
+  </ItemGroup>
+
+</Project>