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_deck_load.gd

@@ -0,0 +1,9 @@
+extends SceneTree
+
+func _initialize():
+	var d = load("res://src/core/deck.gd")
+	if d:
+		print("Deck loaded: ", d)
+	else:
+		print("FAIL: Deck not found")
+	quit()