feat(client): protocol v6 bridge — stance, inventory, input mapping

Upgrade client protocol bridge from v5 to v6 to match server.
Adds player_stance (4 variants) and player_inventory decode to
ObserverSnapshot. Adds TOGGLE_STANCE_UP/DOWN to InputMapper.
Includes 25 gdUnit4 tests for v6 decode + server serialization
test gap fix.

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

client/scripts/autoloads/input_mapper.gd

@@ -6,7 +6,8 @@ extends Node
 enum Action {
 	MOVE_NORTH, MOVE_NORTHEAST, MOVE_EAST, MOVE_SOUTHEAST,
 	MOVE_SOUTH, MOVE_SOUTHWEST, MOVE_WEST, MOVE_NORTHWEST,
-	INTERACT, USE_PERCEPTION_MODE, OPEN_MENU, PAUSE
+	INTERACT, USE_PERCEPTION_MODE, OPEN_MENU, PAUSE,
+	TOGGLE_STANCE_UP, TOGGLE_STANCE_DOWN,
 }
 
 var input_queue: Array[Dictionary] = []
@@ -40,6 +41,10 @@ func _unhandled_input(event: InputEvent) -> void:
 		action = Action.OPEN_MENU
 	elif event.is_action_pressed("pause"):
 		action = Action.PAUSE
+	elif event.is_action_pressed("stance_up"):
+		action = Action.TOGGLE_STANCE_UP
+	elif event.is_action_pressed("stance_down"):
+		action = Action.TOGGLE_STANCE_DOWN
 
 	# Queue the action if valid
 	if action != -1: