fix(client): visual_capture Protocol decode in -s mode

visual_capture.gd runs via `godot -s`, where the `class_name` registry isn't
populated — so the bare `Protocol.decode_snapshot()` reference failed to
compile, breaking ALL --screenshot/--movie/golden captures (not just the
replay scenarios that use it). Instantiate the script and call the static
decoder on the instance (then free), matching the file's existing -s-mode
load() workaround for VisualScenarios.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

client/tests/visual_capture.gd

@@ -184,7 +184,12 @@ func _run_scenario(_main_node: Node) -> void:
 		rf.close()
 		# Decode through Protocol.decode_snapshot() — same as live IPC receive path.
 		# This exercises: msgpack decode → entity decode → tile_kind→type mapping → etc.
-		var replay_data: Variant = Protocol.decode_snapshot(replay_bytes)
+		# class_name `Protocol` isn't resolvable in -s mode (see header note);
+		# instantiate the script and call the static decoder on the instance,
+		# then free it — the decoded Dictionary is independent of the node.
+		var protocol_node: Node = load("res://scripts/protocol/protocol.gd").new()
+		var replay_data: Variant = protocol_node.decode_snapshot(replay_bytes)
+		protocol_node.free()
 		if replay_data == null or not replay_data is Dictionary:
 			push_error("visual_capture: Protocol.decode_snapshot failed for %s" % abs_path)
 			quit(1)