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/Rendering/RenderBootstrap.cs

@@ -22,8 +22,21 @@ public sealed record RenderStack(
     Wb.WbDrawDispatcher DrawDispatcher,
     SceneLightingUboBinding LightingUbo,
     AcDream.App.UI.UiHost UiHost,
-    AcDream.App.UI.UiDatFont? VitalsDatFont)
+    AcDream.App.UI.UiDatFont? VitalsDatFont) : System.IDisposable
 {
+    /// <summary>Dispose the GL pieces this stack OWNS (everything created in
+    /// <see cref="RenderBootstrap.Create"/>). <see cref="Dats"/> + <see cref="Gl"/> are caller-owned
+    /// and NOT disposed here. Called once at studio teardown.</summary>
+    public void Dispose()
+    {
+        DrawDispatcher.Dispose();
+        MeshAdapter.Dispose();
+        TextureCache.Dispose();
+        MeshShader.Dispose();
+        LightingUbo.Dispose();
+        UiHost.Dispose();
+    }
+
     /// <summary>
     /// Resolves a sprite id (0x06xxxxxx) to a (GL handle, width, height) triple.
     /// Copied verbatim from GameWindow's ResolveChrome closure — it calls