feat(client): enforce PROTOCOL_VERSION check in snapshot decode

Client now rejects snapshots where version != PROTOCOL_VERSION (4).
Returns null with error log on mismatch. Test snapshot updated to
use Protocol.PROTOCOL_VERSION and v4 game_time format (tick_rate
replaces paused field).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/scripts/autoloads/sim_bridge.gd

@@ -296,12 +296,12 @@ func _test_snapshot() -> Dictionary:
 
 	return {
 		"tick": _test_tick,
-		"version": 2,
+		"version": Protocol.PROTOCOL_VERSION,
 		"game_time": {
 			"day": 0,
 			"time_of_day": _test_tick * 10,
 			"day_phase": "Morning",
-			"paused": false,
+			"tick_rate": "Full",
 		},
 		"player_facing": _test_facing,
 		"entities": entities,