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.

scenes/prop_attach_preview.tscn

@@ -0,0 +1,9 @@
+[gd_scene load_steps=2 format=3]
+
+[ext_resource type="Script" path="res://scripts/prop_attach_preview.gd" id="1"]
+
+[node name="PropAttachPreview" type="PanelContainer"]
+custom_minimum_size = Vector2(360, 0)
+size_flags_horizontal = 3
+size_flags_vertical = 3
+script = ExtResource("1")