fix(studio): Task 3 review — UiRoot.Pick, RenderStack IDisposable, dt cleanup

Code-review follow-ups to the ImGui inspector:
- Add public UiRoot.Pick(x,y) over the private HitTestTopDown (honors
  Z-order + modal exclusivity); StudioWindow uses it instead of a manual
  UiElement.HitTest with subtracted ScreenPosition.
- RenderStack : IDisposable — disposes the GL pieces it owns in one place;
  StudioWindow OnClosing + Dispose both call _stack?.Dispose(), closing the
  error-path leak (only UiHost was disposed on the Dispose-without-OnClosing
  path).
- Drop the stale _dt field; OnRender passes its own dt to Tick + BeginFrame.
- Fix a stale PanelFbo comment.

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

src/AcDream.App/UI/UiRoot.cs

@@ -655,6 +655,10 @@ public sealed class UiRoot : UiElement
         return (null, 0, 0);
     }
 
+    /// <summary>Public hit-test for tooling (the UI Studio inspector): the topmost element under
+    /// (x,y) in root space, honoring modal exclusivity + Z-order. Wraps the private HitTestTopDown.</summary>
+    public UiElement? Pick(int x, int y) => HitTestTopDown(x, y).element;
+
     private static UiElement? FindWindow(UiElement? e)
     {
         while (e is not null)