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,

client/scripts/protocol/protocol.gd

@@ -9,6 +9,10 @@ class_name Protocol
 ##   Unit enum variants (no data) → bare strings ("MoveNorth", "Npc")
 ##   Data enum variants → single-element maps ({"UsePerceptionMode": "thermal"})
 
+## Protocol version — must match server PROTOCOL_VERSION in bridge/types.rs.
+## Reject snapshots where version != this value.
+const PROTOCOL_VERSION: int = 4
+
 
 # -- Decode: bytes from server → GDScript types --------------------------------
 
@@ -27,6 +31,12 @@ static func decode_snapshot(bytes: PackedByteArray) -> Variant:
 		push_error("Protocol: snapshot missing required fields")
 		return null
 
+	# Version check: reject snapshots from incompatible server
+	var version: Variant = raw.get("version")
+	if version != PROTOCOL_VERSION:
+		push_error("Protocol: version mismatch (got %s, expected %s). Server and client are out of sync." % [version, PROTOCOL_VERSION])
+		return null
+
 	var entities: Array[Dictionary] = []
 	var raw_entities: Array = raw["entities"]
 	var dropped := 0
@@ -44,8 +54,7 @@ static func decode_snapshot(bytes: PackedByteArray) -> Variant:
 	# in any realistic scenario (would require ~29 billion years at 10 ticks/game-minute per D-031).
 	var tick: int = raw["tick"]
 
-	# v2 fields — optional for backward compatibility
-	var version: Variant = raw.get("version")
+	# version already checked above; game_time for HUD display
 	var game_time: Variant = raw.get("game_time")
 
 	# player_facing: FacingDirection is a unit enum → bare string in rmp_serde