test(ui): AcDream.UI.Abstractions unit tests (11 tests green)

Covers the pure-logic surface of commit 1:

  VitalsVMTests
    * HealthPercent reads from CombatState.GetHealthPercent
    * safe-default 1.0 when GUID unknown / not yet set / never updated
    * SetLocalPlayerGuid reroutes lookup to the new guid (no stale cache)
    * StaminaPercent / ManaPercent are null for D.2a scope
    * ctor throws ArgumentNullException on null CombatState

  PanelContextTests
    * record-struct fields round-trip
    * value-based equality

  NullCommandBusTests
    * Publish accepts any record type without throwing
    * Instance is a true singleton

csproj template mirrors AcDream.Core.Tests (xUnit 2.9.3, Test.Sdk 17.14.1,
runner 3.1.4, coverlet 6.0.4, implicit Using Xunit). References only
AcDream.UI.Abstractions — no runtime / GL dependency, tests run fast.

tests/AcDream.UI.Abstractions.Tests/AcDream.UI.Abstractions.Tests.csproj

@@ -0,0 +1,25 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="6.0.4" />
+    <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" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\AcDream.UI.Abstractions\AcDream.UI.Abstractions.csproj" />
+  </ItemGroup>
+
+</Project>