feat(topology): add square light canvas

Co-Authored-By: OpenAI Codex <codex@openai.com>

tests/test_guanghan_topology_demo.py

@@ -49,6 +49,13 @@ class GuanghanTopologySourceTests(unittest.TestCase):
         self.assertIn("decorationStyles", self.html)
         self.assertIn("legendItems", self.html)
 
+    def test_canvas_is_square_and_page_defaults_to_light(self) -> None:
+        self.assertIn('viewBox="0 0 1900 1900"', self.html)
+        self.assertIn("payload.layout.canvasWidth", self.html)
+        self.assertIn("payload.layout.canvasHeight", self.html)
+        self.assertIn("color-scheme: light", self.html)
+        self.assertNotIn("color-scheme: dark", self.html)
+
     def test_business_visual_semantics_are_not_css_hardcoded(self) -> None:
         forbidden = [
             ".connection.structural",