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/Studio/PanelFbo.cs

@@ -56,7 +56,7 @@ public sealed unsafe class PanelFbo : IDisposable
         _gl.BindFramebuffer(FramebufferTarget.Framebuffer, _fbo);
         _gl.Viewport(0, 0, (uint)width, (uint)height);
         _gl.Disable(EnableCap.ScissorTest);
-        _gl.ClearColor(0.18f, 0.18f, 0.18f, 1f);   // dark bg matching the old direct draw
+        _gl.ClearColor(0.18f, 0.18f, 0.18f, 1f);   // opaque dark-grey canvas background (the FBO IS the canvas)
         _gl.ClearDepth(1.0);
         _gl.DepthMask(true);
         _gl.Clear(ClearBufferMask.ColorBufferBit | ClearBufferMask.DepthBufferBit);