feat(plugins): complete Campaign LA5 cross-host hosting

src/AcDream.Plugin.Abstractions/IPluginHost.cs

@@ -7,6 +7,14 @@ namespace AcDream.Plugin.Abstractions;
 /// </summary>
 public interface IPluginHost
 {
+    /// <summary>
+    /// <see langword="true"/> when <see cref="Ui"/> registrations can be
+    /// projected by this host. No-window hosts return <see langword="false"/>
+    /// and expose <see cref="NoOpUiRegistry.Instance"/> so a plugin may keep
+    /// one code path while deliberately omitting presentation work.
+    /// </summary>
+    bool HasUi { get; }
+
     IPluginLogger Log { get; }
     IGameState State { get; }
     IEvents Events { get; }

src/AcDream.Plugin.Abstractions/IUiRegistry.cs

@@ -3,8 +3,10 @@ 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.
+/// registers it from <c>Enable()</c>. Graphical hosts buffer registrations until
+/// their retained UI exists. A host whose <see cref="IPluginHost.HasUi"/> is
+/// <see langword="false"/> exposes <see cref="NoOpUiRegistry.Instance"/> and
+/// intentionally discards registrations.
 /// </summary>
 public interface IUiRegistry
 {
@@ -12,3 +14,21 @@ public interface IUiRegistry
     /// <param name="binding">Object whose properties the markup's {Bindings} resolve against.</param>
     void AddMarkupPanel(string markupPath, object binding);
 }
+
+/// <summary>
+/// BCL-only UI sink for no-window plugin hosts. It intentionally retains
+/// neither markup paths nor binding objects, so a UI registration cannot keep
+/// a plugin assembly alive after its collectible load context is unloaded.
+/// </summary>
+public sealed class NoOpUiRegistry : IUiRegistry
+{
+    public static NoOpUiRegistry Instance { get; } = new();
+
+    private NoOpUiRegistry()
+    {
+    }
+
+    public void AddMarkupPanel(string markupPath, object binding)
+    {
+    }
+}