feat: Implement command-line scene loading and PLAY-on-launch functionality

scripts/cli_scene_launcher.gd

@@ -0,0 +1,47 @@
+class_name CliSceneLauncher
+extends RefCounted
+## Pure command-line argument parser for the Sandbox Stage scene loader.
+## No scene-tree or OS access, so it is directly unit-testable. Parses
+## OS.get_cmdline_user_args() output into a structured {scene_path, play, error}
+## result; the caller (SandboxStage) is responsible for the error dialog and
+## autosave policy.
+
+static func parse(args: Array) -> Dictionary:
+	var scene_path := ""
+	var play := false
+	var used_scene_flag := false
+	var used_positional := false
+	var error := ""
+	var i := 0
+	while i < args.size():
+		var token := String(args[i])
+		if token == "--play":
+			play = true
+		elif token == "--scene":
+			used_scene_flag = true
+			if i + 1 >= args.size():
+				return {"scene_path": "", "play": play, "error": "--scene requires a path."}
+			var value := String(args[i + 1])
+			if value.is_empty():
+				return {"scene_path": "", "play": play, "error": "--scene requires a path."}
+			scene_path = value
+			i += 1
+		elif token.begins_with("--scene="):
+			used_scene_flag = true
+			var value := token.substr("--scene=".length())
+			if value.is_empty():
+				return {"scene_path": "", "play": play, "error": "--scene requires a path."}
+			scene_path = value
+		elif token.begins_with("--"):
+			return {"scene_path": "", "play": play, "error": "Unknown option: %s" % token}
+		else:
+			if used_positional:
+				return {"scene_path": "", "play": play, "error": "Specify the scene with either --scene or a single positional path."}
+			used_positional = true
+			scene_path = token
+		i += 1
+	if used_scene_flag and used_positional:
+		error = "Specify the scene with either --scene or a single positional path."
+	elif play and scene_path.is_empty():
+		error = "--play requires --scene <path>."
+	return {"scene_path": scene_path, "play": play, "error": error}