fix: revert // to / in deck.gd (GDScript 4 does not support // operator)

The INTEGER_DIVISION warning in deck.gd:19 is a false positive - both
operands are int, so / performs integer division correctly in GDScript 4.
Using // caused a parse error (Expected expression after '/' operator),
which cascaded to GameController/TrainingController resolution failure.

src/core/deck.gd

@@ -16,7 +16,7 @@ static func _rank_for(original_id: int) -> int:
 		return 15
 	if original_id == 53:
 		return 16
-	return 2 + (original_id // 4)
+	return 2 + (original_id / 4)
 
 static func create(seed: int = -1) -> Deck:
 	var d := Deck.new()