Implement Phase 19 updates and user/editor verification

- Updated architectural specifications for Phase 19 scenery, backgrounds, and surface finishes to reflect implementation status and user verification results.
- Revised UI/UX specifications for background ground and wall color, plain background, and starter scene composition to indicate successful implementation and verification.
- Enhanced scenery controller and sandbox stage scripts to improve eraser functionality and user interactions.
- Adjusted test cases to align with the latest changes in scenery and background implementations, ensuring all checks pass successfully.
- Added new buttons for wall area erasure in the scenery capture bar and updated layout handling for better user experience.

content/stages/fps.scene

@@ -1,13 +1,15 @@
 {
-	"version": "1.6",
+	"version": "1.7",
 	"scene_name": "Untitled",
 	"sky_id": "sky_sunset",
+	"background_kind": "plain",
+	"plain_color": "#FFFFFF",
 	"camera": {
 		"position": {
-			"x": -85.0290832519531,
-			"y": -612.394409179688
+			"x": 1676.29174804688,
+			"y": -39.946044921875
 		},
-		"zoom": 0.732050120830536
+		"zoom": 0.310460686683655
 	},
 	"objects": [
 		{
@@ -409,8 +411,8 @@
 	],
 	"event_rules": [],
 	"metadata": {
-		"created_unix": 1792467978.761,
-		"modified_unix": 1792467978.761,
+		"created_unix": 1792467978,
+		"modified_unix": 1792700708.001,
 		"autosave": false,
 		"source_path": ""
 	}