Add regression tests for prop attachment, centering, and editor UX

- Implemented test_phase9a_prop_attach.gd to validate anchor schema, alignment math, and preview isolation.
- Implemented test_phase9b_prop_centering.gd to check canonical origin presentation, bounds, centering, and save threshold.
- Implemented test_phase9c_prop_editor_ux.gd to assess the user experience in the prop editor, including anchor renaming and UI responsiveness.
- Added corresponding .uid files for each test.
- Created sync_sources.py script to manage markdown file synchronization with the notebook.
- Added .gitignore for virtual environment files.

prop_theme.json

@@ -41,7 +41,8 @@
       "text": "#F2F2F2",
       "text_muted": "#9A9A9A",
       "border": "#FFFFFF33",
-      "accent": "#4FA3FF"
+      "accent": "#4FA3FF",
+      "inspector_background": "#262C39"
     }
   }
 }