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>

src/AcDream.App/Plugins/AppPluginHost.cs

@@ -4,14 +4,16 @@ namespace AcDream.App.Plugins;
 
 public sealed class AppPluginHost : IPluginHost
 {
-    public AppPluginHost(IPluginLogger log, IGameState state, IEvents events)
+    public AppPluginHost(IPluginLogger log, IGameState state, IEvents events, IUiRegistry ui)
     {
         Log = log;
         State = state;
         Events = events;
+        Ui = ui;
     }
 
     public IPluginLogger Log { get; }
     public IGameState State { get; }
     public IEvents Events { get; }
+    public IUiRegistry Ui { get; }
 }

src/AcDream.App/Plugins/BufferedUiRegistry.cs

@@ -0,0 +1,27 @@
+using System.Collections.Generic;
+using AcDream.Plugin.Abstractions;
+
+namespace AcDream.App.Plugins;
+
+/// <summary>
+/// Buffers plugin <see cref="IUiRegistry.AddMarkupPanel"/> calls (which run in
+/// Program.cs before the GL window opens) until GameWindow drains them into the
+/// UiHost tree after construction.
+/// </summary>
+public sealed class BufferedUiRegistry : IUiRegistry
+{
+    public readonly record struct Pending(string MarkupPath, object Binding);
+
+    private readonly List<Pending> _pending = new();
+
+    public void AddMarkupPanel(string markupPath, object binding)
+        => _pending.Add(new Pending(markupPath, binding));
+
+    /// <summary>Return + clear all buffered registrations.</summary>
+    public IReadOnlyList<Pending> Drain()
+    {
+        var copy = _pending.ToArray();
+        _pending.Clear();
+        return copy;
+    }
+}

src/AcDream.App/Program.cs

@@ -23,7 +23,8 @@ var runtimeOptions = RuntimeOptions.FromEnvironment(datDir);
 
 var worldGameState = new AcDream.Core.Plugins.WorldGameState();
 var worldEvents = new AcDream.Core.Plugins.WorldEvents();
-var host = new AppPluginHost(new SerilogAdapter(Log.Logger), worldGameState, worldEvents);
+var uiRegistry = new AcDream.App.Plugins.BufferedUiRegistry();
+var host = new AppPluginHost(new SerilogAdapter(Log.Logger), worldGameState, worldEvents, uiRegistry);
 
 var pluginsDir = Path.Combine(AppContext.BaseDirectory, "plugins");
 Log.Information("scanning plugins in {PluginsDir}", pluginsDir);
@@ -56,7 +57,7 @@ try
         catch (Exception ex) { Log.Error(ex, "plugin enable failed: {Id}", plugin.Manifest.Id); }
     }
 
-    using var window = new GameWindow(runtimeOptions, worldGameState, worldEvents);
+    using var window = new GameWindow(runtimeOptions, worldGameState, worldEvents, uiRegistry);
     window.Run();
 }
 finally

src/AcDream.App/Rendering/GameWindow.cs

@@ -614,6 +614,8 @@ public sealed class GameWindow : IDisposable
     private AcDream.UI.Abstractions.Panels.Vitals.VitalsVM? _vitalsVm;
     // Phase D.2b — retail-look UI tree (dormant UiHost wired here). Null unless ACDREAM_RETAIL_UI=1.
     private AcDream.App.UI.UiHost? _uiHost;
+    // Phase D.2b Task 9 — plugin UI registrations buffered before OnLoad; drained in OnLoad.
+    private readonly AcDream.App.Plugins.BufferedUiRegistry? _uiRegistry;
     // Phase I.2: ImGui debug panel ViewModel. Lives for as long as
     // _panelHost does. Self-subscribes to CombatState in its ctor, so
     // disposing isn't required (panel host holds the only ref).
@@ -864,12 +866,14 @@ public sealed class GameWindow : IDisposable
     private int _liveAnimRejectSingleFrame;
     private int _liveAnimRejectPartFrames;
 
-    public GameWindow(AcDream.App.RuntimeOptions options, WorldGameState worldGameState, WorldEvents worldEvents)
+    public GameWindow(AcDream.App.RuntimeOptions options, WorldGameState worldGameState, WorldEvents worldEvents,
+        AcDream.App.Plugins.BufferedUiRegistry? uiRegistry = null)
     {
         _options = options ?? throw new System.ArgumentNullException(nameof(options));
         _datDir = options.DatDir;
         _worldGameState = worldGameState;
         _worldEvents = worldEvents;
+        _uiRegistry = uiRegistry;
         SpellBook = new AcDream.Core.Spells.Spellbook(SpellTable);
         LocalPlayer = new AcDream.Core.Player.LocalPlayerState(SpellBook);
     }
@@ -1758,6 +1762,28 @@ public sealed class GameWindow : IDisposable
             var panel = AcDream.App.UI.MarkupDocument.Build(vitalsXml, _vitalsVm!, ResolveChrome, controls);
             _uiHost.Root.AddChild(panel);
             Console.WriteLine("[D.2b] retail UI active — vitals panel from vitals.xml markup.");
+
+            // Drain plugin-registered markup panels (buffered before the GL
+            // window opened) into the same UiRoot tree. A faulty plugin markup
+            // file is isolated — logged + skipped, never crashes the client.
+            if (_uiRegistry is not null)
+            {
+                foreach (var p in _uiRegistry.Drain())
+                {
+                    try
+                    {
+                        string pluginXml = System.IO.File.ReadAllText(p.MarkupPath);
+                        var pluginPanel = AcDream.App.UI.MarkupDocument.Build(
+                            pluginXml, p.Binding, ResolveChrome, controls);
+                        _uiHost.Root.AddChild(pluginPanel);
+                        Console.WriteLine($"[D.2b] plugin UI panel loaded: {p.MarkupPath}");
+                    }
+                    catch (Exception ex)
+                    {
+                        Console.WriteLine($"[D.2b] plugin UI panel '{p.MarkupPath}' failed to load: {ex.Message}");
+                    }
+                }
+            }
         }
 
         // Phase N.4+N.5 — WB rendering pipeline foundation. The modern path is

src/AcDream.Plugin.Abstractions/IPluginHost.cs

@@ -10,4 +10,5 @@ public interface IPluginHost
     IPluginLogger Log { get; }
     IGameState State { get; }
     IEvents Events { get; }
+    IUiRegistry Ui { get; }
 }

src/AcDream.Plugin.Abstractions/IUiRegistry.cs

@@ -0,0 +1,14 @@
+namespace AcDream.Plugin.Abstractions;
+
+/// <summary>
+/// Plugin-facing UI registration. A plugin ships a markup file (KSML-style) +
+/// a binding object exposing the data properties the markup binds to, and
+/// registers it from <c>Enable()</c>. Calls made before the GL window opens are
+/// buffered and drained once the UI host exists.
+/// </summary>
+public interface IUiRegistry
+{
+    /// <param name="markupPath">Absolute path to the plugin's panel markup file.</param>
+    /// <param name="binding">Object whose properties the markup's {Bindings} resolve against.</param>
+    void AddMarkupPanel(string markupPath, object binding);
+}

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