Automate WinUtil title screen generation (#4965)

* feat: add title screen generation tooling

Add local automation for capturing WinUtil in Light and Dark themes
and generating the composite title-screen image.

* ci: automate title screen updates

Add a manual Windows workflow that compiles WinUtil, generates the
Light and Dark composite, and opens an image-only pull request.

* docs: update AGENTS.md and SPEC.md with title-screen generation details

* fix: address review comments

Tested image is correct, SHA-256 hash is identical to the previously verified output.

* fix: deselect bitmap before reading capture pixels

tools/title-screen/test_create_composite.py

@@ -0,0 +1,45 @@
+"""Smoke tests for the WinUtil title-screen compositor."""
+
+import tempfile
+import unittest
+from pathlib import Path
+
+from PIL import Image, ImageDraw
+
+from create_composite import BORDER_COLOR, create_composite
+
+
+class CreateCompositeTests(unittest.TestCase):
+    def test_creates_only_requested_composite(self):
+        with tempfile.TemporaryDirectory(prefix="winutil-composite-test-") as temp_dir:
+            work_dir = Path(temp_dir)
+            dark_path = work_dir / "dark.png"
+            light_path = work_dir / "light.png"
+            output_path = work_dir / "title-screen.png"
+
+            self._create_capture(dark_path, (32, 34, 36, 255))
+            self._create_capture(light_path, (235, 237, 239, 255))
+
+            result = create_composite(dark_path, light_path, output_path)
+
+            self.assertEqual(result, output_path)
+            self.assertEqual(
+                {path.name for path in work_dir.glob("*.png")},
+                {"dark.png", "light.png", "title-screen.png"},
+            )
+            with Image.open(output_path) as composite:
+                self.assertEqual(composite.size, (64, 40))
+                self.assertEqual(composite.getpixel((0, 0)), BORDER_COLOR)
+                self.assertEqual(composite.getpixel((5, 20)), (235, 237, 239, 255))
+                self.assertEqual(composite.getpixel((58, 20)), (32, 34, 36, 255))
+
+    @staticmethod
+    def _create_capture(path: Path, content_color: tuple[int, int, int, int]):
+        image = Image.new("RGBA", (70, 46), (0, 0, 0, 255))
+        draw = ImageDraw.Draw(image)
+        draw.rectangle((3, 3, 66, 42), fill=content_color)
+        image.save(path, "PNG")
+
+
+if __name__ == "__main__":
+    unittest.main()