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")

scenes/prop_editor.tscn

@@ -28,7 +28,7 @@ script = ExtResource("1_editor_script")
 layout_mode = 1
 anchors_preset = 0
 anchor_right = 1.0
-offset_bottom = 32.0
+offset_bottom = 44.0
 grow_horizontal = 2
 
 [node name="MenuBar" type="MenuBar" parent="TopRow"]
@@ -45,8 +45,8 @@ theme_override_constants/separation = 8
 layout_mode = 1
 anchors_preset = 0
 anchor_right = 1.0
-offset_top = 32.0
-offset_bottom = 64.0
+offset_top = 44.0
+offset_bottom = 88.0
 grow_horizontal = 2
 
 [node name="NameLabel" type="Label" parent="TopBar"]
@@ -61,12 +61,13 @@ layout_mode = 2
 size_flags_horizontal = 3
 placeholder_text = "Unnamed Prop"
 
-[node name="MainLayout" type="HBoxContainer" parent="."]
+[node name="MainLayout" type="HSplitContainer" parent="."]
+unique_name_in_owner = true
 layout_mode = 1
 anchors_preset = 15
 anchor_right = 1.0
 anchor_bottom = 1.0
-offset_top = 68.0
+offset_top = 92.0
 offset_bottom = -28.0
 grow_horizontal = 2
 grow_vertical = 2
@@ -78,6 +79,7 @@ layout_mode = 2
 size_flags_horizontal = 3
 size_flags_vertical = 3
 size_flags_stretch_ratio = 2.0
+custom_minimum_size = Vector2(260, 0)
 theme_override_styles/panel = SubResource("StyleBox_canvas")
 
 [node name="CanvasArea" type="VBoxContainer" parent="MainLayout/CanvasPanel"]
@@ -89,7 +91,19 @@ unique_name_in_owner = true
 layout_mode = 2
 theme_override_constants/separation = 8
 
-[node name="PropCanvas" type="Control" parent="MainLayout/CanvasPanel/CanvasArea"]
+[node name="AttachTools" type="FlowContainer" parent="MainLayout/CanvasPanel/CanvasArea"]
+unique_name_in_owner = true
+layout_mode = 2
+theme_override_constants/separation = 12
+
+[node name="CanvasWorkspace" type="HSplitContainer" parent="MainLayout/CanvasPanel/CanvasArea"]
+unique_name_in_owner = true
+layout_mode = 2
+size_flags_horizontal = 3
+size_flags_vertical = 3
+split_offset = 520
+
+[node name="PropCanvas" type="Control" parent="MainLayout/CanvasPanel/CanvasArea/CanvasWorkspace"]
 unique_name_in_owner = true
 layout_mode = 2
 size_flags_horizontal = 3
@@ -98,13 +112,18 @@ mouse_filter = 0
 clip_contents = true
 script = ExtResource("2_canvas_script")
 
-[node name="InspectorColumn" type="VBoxContainer" parent="MainLayout"]
+[node name="InspectorColumn" type="PanelContainer" parent="MainLayout"]
 unique_name_in_owner = true
 layout_mode = 2
 size_flags_vertical = 3
 custom_minimum_size = Vector2(300, 0)
+size_flags_horizontal = 3
 
-[node name="InspectorList" type="VBoxContainer" parent="MainLayout/InspectorColumn"]
+[node name="InspectorScroll" type="ScrollContainer" parent="MainLayout/InspectorColumn"]
+layout_mode = 2
+size_flags_vertical = 3
+
+[node name="InspectorList" type="VBoxContainer" parent="MainLayout/InspectorColumn/InspectorScroll"]
 unique_name_in_owner = true
 layout_mode = 2
 size_flags_vertical = 3