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.

scripts/prop_theme.gd

@@ -42,7 +42,7 @@ const DEFAULT_CONFIG: Dictionary = {
 	"ui": {
 		"fonts": {"body": 18, "header": 24, "menu": 22, "menu_bar": 22, "mode": 26, "status": 18, "prop_name": 20, "context_menu": 22},
 		"hit_targets": {"button_min_height": 36, "row_min_height": 36, "mode_button_min_height": 56},
-		"colors": {"text": "#F2F2F2", "text_muted": "#9A9A9A", "border": "#FFFFFF33", "accent": "#4FA3FF"},
+		"colors": {"text": "#F2F2F2", "text_muted": "#9A9A9A", "border": "#FFFFFF33", "accent": "#4FA3FF", "inspector_background": "#262C39"},
 	},
 }
 
@@ -218,6 +218,10 @@ func accent_color() -> Color:
 	return color("ui.colors.accent", Color("#4FA3FF"))
 
 
+func inspector_background_color() -> Color:
+	return color("ui.colors.inspector_background", Color("#262C39"))
+
+
 func invalidate_caches() -> void:
 	_warned.clear()