feat(studio): headless --screenshot mode (render panel to PNG + exit)

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>

src/AcDream.App/Studio/PanelFbo.cs

@@ -67,6 +67,34 @@ public sealed unsafe class PanelFbo : IDisposable
         return _colorTex;
     }
 
+    /// <summary>
+    /// Read the FBO color attachment back to CPU as a flat RGBA8 byte array.
+    /// Must be called AFTER <see cref="Render"/> for the same <paramref name="width"/> and
+    /// <paramref name="height"/> (so the FBO exists and is the right size).
+    ///
+    /// <para>FBO origin is bottom-left (GL convention). The caller is responsible for
+    /// flipping rows vertically before saving as a top-left-origin image format (PNG).</para>
+    ///
+    /// <para>Returns an empty array when the FBO is not ready.</para>
+    /// </summary>
+    public unsafe byte[] ReadColorRgba(int width, int height)
+    {
+        if (_fbo == 0 || width <= 0 || height <= 0) return Array.Empty<byte>();
+
+        int byteCount = width * height * 4;
+        var buf = new byte[byteCount];
+
+        _gl.BindFramebuffer(FramebufferTarget.Framebuffer, _fbo);
+        fixed (byte* p = buf)
+        {
+            _gl.ReadPixels(0, 0, (uint)width, (uint)height,
+                PixelFormat.Rgba, PixelType.UnsignedByte, p);
+        }
+        _gl.BindFramebuffer(FramebufferTarget.Framebuffer, 0);
+
+        return buf;
+    }
+
     // ── FBO lifecycle (mirrors PaperdollViewportRenderer.EnsureFramebuffer) ──────
 
     private void EnsureFramebuffer(int width, int height)