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.

tests/test_training_controller.gd

@@ -0,0 +1,14 @@
+extends SceneTree
+
+func _initialize():
+	var t = load("res://src/game/game_controller.gd")
+	if t:
+		print("GameController loaded: ", t)
+	else:
+		print("FAIL: GameController not found")
+	var tc = load("res://src/game/training_controller.gd")
+	if tc:
+		print("TrainingController loaded: ", tc)
+	else:
+		print("FAIL: TrainingController not found")
+	quit()