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_file.gd

@@ -9,6 +9,7 @@ extends RefCounted
 
 const PropBlockScript := preload("res://scripts/prop_block.gd")
 const TerrainUtilsScript := preload("res://scripts/terrain_utils.gd")
+const PropAnchorDataScript := preload("res://scripts/prop_anchor_data.gd")
 
 const FILE_VERSION: String = "1.0"
 const FILE_EXT: String = ".prop"
@@ -61,7 +62,7 @@ static func sanitize_shape(shape: Dictionary, grid_size: float = TerrainUtilsScr
 	return result
 
 
-static func to_file_dict(prop_name: String, material_preset: int, shapes: Array[Dictionary]) -> Dictionary:
+static func to_file_dict(prop_name: String, material_preset: int, shapes: Array[Dictionary], anchors: Dictionary = {}) -> Dictionary:
 	var serialized: Array = []
 	for shape: Dictionary in shapes:
 		serialized.append(_serialize_shape(shape))
@@ -71,6 +72,7 @@ static func to_file_dict(prop_name: String, material_preset: int, shapes: Array[
 		"prop_name": prop_name,
 		"material_preset": preset_to_string(material_preset),
 		"shapes": serialized,
+		"anchors": PropAnchorDataScript.serialize_anchors(anchors),
 		"metadata": {"created_at": now, "modified_at": now},
 	}
 
@@ -105,6 +107,7 @@ static func from_file_dict(data: Variant) -> Dictionary:
 		"prop_name": str(dict.get("prop_name", "")),
 		"material_preset": preset,
 		"shapes": shapes,
+		"anchors": PropAnchorDataScript.normalize_anchors(dict.get("anchors", {})),
 	}