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/NullCommandBusTests.cs

@@ -0,0 +1,22 @@
+namespace AcDream.UI.Abstractions.Tests;
+
+public sealed class NullCommandBusTests
+{
+    private sealed record FakeCmd(int Value);
+
+    [Fact]
+    public void Publish_DoesNotThrow_OnAnyRecordType()
+    {
+        var bus = NullCommandBus.Instance;
+
+        bus.Publish(new FakeCmd(42));
+        bus.Publish("a string command");
+        bus.Publish(12345);
+    }
+
+    [Fact]
+    public void Instance_IsSingleton()
+    {
+        Assert.Same(NullCommandBus.Instance, NullCommandBus.Instance);
+    }
+}