feat(D.2b): IUiRegistry plugin UI surface + buffered drain into UiHost

Adds the plugin-facing UI registration surface (Task 9, final D.2b task).
Plugins call host.Ui.AddMarkupPanel(path, binding) from Enable(); calls are
buffered in BufferedUiRegistry before the GL window opens, then drained into
UiHost.Root in GameWindow.OnLoad inside the RetailUi block after the first-
party vitals panel. Faulty plugin markup is isolated (try/catch per panel,
logged + skipped). IPluginHost.Ui added; AppPluginHost wired; StubHost in
Core.Tests updated; BufferedUiRegistryTests confirms drain-once semantics.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/AcDream.App.Tests/Plugins/BufferedUiRegistryTests.cs

@@ -0,0 +1,21 @@
+using AcDream.App.Plugins;
+
+namespace AcDream.App.Tests.Plugins;
+
+public class BufferedUiRegistryTests
+{
+    [Fact]
+    public void Drain_YieldsBufferedRegistrationsOnceThenEmpty()
+    {
+        var reg = new BufferedUiRegistry();
+        reg.AddMarkupPanel("a.xml", new object());
+        reg.AddMarkupPanel("b.xml", new object());
+
+        var drained = reg.Drain();
+        Assert.Equal(2, drained.Count);
+        Assert.Equal("a.xml", drained[0].MarkupPath);
+        Assert.Equal("b.xml", drained[1].MarkupPath);
+
+        Assert.Empty(reg.Drain());   // consumed
+    }
+}

tests/AcDream.Core.Tests/Plugins/PluginLoaderTests.cs

@@ -30,6 +30,12 @@ public class PluginLoaderTests
         public IPluginLogger Log { get; } = new StubLogger();
         public IGameState State { get; } = new StubState();
         public IEvents Events { get; } = new StubEvents();
+        public IUiRegistry Ui { get; } = new StubUiRegistry();
+    }
+
+    private sealed class StubUiRegistry : IUiRegistry
+    {
+        public void AddMarkupPanel(string markupPath, object binding) { }
     }
 
     private sealed class StubLogger : IPluginLogger