Add `--screenshot <path>` to the UI Studio: when set the window is
created hidden (WindowOptions.IsVisible = false), the panel is loaded
and rendered into PanelFbo on the first OnRender tick, pixels are read
back with the new PanelFbo.ReadColorRgba(), rows are flipped (GL
bottom-left → PNG top-left), and the result is saved via ImageSharp
SaveAsPng before calling _window.Close().
Render size is derived from the loaded root's Width/Height (clamped
256–2048 px); falls back to 1280×720 when the root has no explicit
size. In headless mode ImGui, the inspector, and input wiring are
all skipped — only PanelFbo is created. Interactive path is
unchanged. SixLabors.ImageSharp 3.1.12 was already a direct dep of
AcDream.App (Phase O-T7); no new PackageReference needed.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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