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.

.godot/editor/script_editor_cache.cfg

@@ -0,0 +1,13 @@
+[res://src/game/training_controller.gd]
+
+state={
+"bookmarks": PackedInt32Array(),
+"breakpoints": PackedInt32Array(),
+"column": 0,
+"folded_lines": PackedInt32Array(),
+"h_scroll_position": 0,
+"row": 1,
+"scroll_position": 1.0,
+"selection": false,
+"syntax_highlighter": "GDScript"
+}