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_rule_engine.gd.uid

@@ -0,0 +1 @@
+uid://ryaamcy0nefy