feat(D.2b): ChatWindowController — bind chat LayoutDesc, place widgets, route chat

Implements Task G2: binds the imported chat LayoutDesc (0x21000006) to live
behavior, the acdream analogue of retail ChatInterface + gmMainChatUI::PostInit.

- UiDatElement: add OnClick hook + OnEvent override so Send/max-min buttons
  can be wired by a controller without needing a dedicated widget type.
- ChatWindowController.Bind: reads transcript (0x10000011) and input
  (0x10000016) rects from the raw ElementInfo tree (factory skips them as
  Type-12/no-media), places UiChatView under the transcript panel and
  UiChatInput under the input bar; replaces the imported scrollbar track
  (0x10000012) with UiChatScrollbar driving UiChatView.Scroll; replaces
  the channel menu placeholder (0x10000014) with UiChannelMenu; wires
  Send button and max/min toggle via the new OnClick hook.
  ChatCommandRouter.Submit routes all input through the existing pipeline.
- 6 smoke tests: Bind returns non-null, Transcript is child of panel,
  Input is child of bar, Input.OnSubmit publishes SendChatCmd, channel
  change updates submit channel, returns null when panels missing.

Build: 0 errors. Test suite: 392 passed / 1 skipped / 0 failed.

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

src/AcDream.App/UI/Layout/UiDatElement.cs

@@ -76,6 +76,17 @@ public sealed class UiDatElement : UiElement
          : _info.StateMedia.TryGetValue("", out var d) ? d
          : (0u, 0);
 
+    /// <summary>Optional click handler. Set by a controller for interactive dat
+    /// elements (e.g. the chat Send / max-min buttons). Requires
+    /// <see cref="UiElement.ClickThrough"/> = false to receive click events.</summary>
+    public Action? OnClick { get; set; }
+
+    public override bool OnEvent(in UiEvent e)
+    {
+        if (e.Type == UiEventType.Click && OnClick is not null) { OnClick(); return true; }
+        return false;
+    }
+
     protected override void OnDraw(UiRenderContext ctx)
     {
         var (file, _) = ActiveMedia();