Add exact arithmetic implementation and tests

- Introduced exact arithmetic with support for large integers and fractions in `exact.gd`, `big_integer.gd`, and `exact_value.gd`.
- Implemented a comprehensive test suite in `test_exact_math.gd` to validate arithmetic operations against expected results.
- Added a README in `math` directory to document the exact arithmetic system and its usage.
- Ensured that all arithmetic operations maintain precision without floating-point errors, using a custom representation for large numbers.
- Included methods for encoding and decoding exact values for serialization.

world/world.tscn

@@ -99,6 +99,7 @@ size_flags_vertical = 3
 script = ExtResource("3_k83x5")
 
 [node name="World" type="VBoxContainer" parent="Stage" unique_id=1878643860]
+visible = false
 layout_mode = 1
 anchors_preset = 10
 anchor_right = 1.0
@@ -1058,10 +1059,9 @@ texture = ExtResource("61_d36xh")
 stretch_mode = 3
 
 [node name="Credits" type="VBoxContainer" parent="Stage" unique_id=785042592]
-visible = false
 layout_mode = 0
-offset_right = 40.0
-offset_bottom = 40.0
+offset_right = 1080.0
+offset_bottom = 1920.0
 script = ExtResource("62_o2i0n")
 
 [node name="TopMenu" type="HBoxContainer" parent="Stage/Credits" unique_id=1983527467]
@@ -1088,15 +1088,20 @@ texture = ExtResource("62_vu83j")
 custom_minimum_size = Vector2(1080, 0)
 layout_mode = 2
 
-[node name="Highlight" type="TextureRect" parent="Stage/Credits/Logo" unique_id=1035898922]
-visible = false
-layout_mode = 2
-texture = ExtResource("64_rur7c")
-
 [node name="Lamp" type="TextureButton" parent="Stage/Credits/Logo" unique_id=1059180893]
 layout_mode = 2
 texture_normal = ExtResource("65_y1rwn")
 
+[node name="Highlight" type="TextureRect" parent="Stage/Credits/Logo" unique_id=1035898922]
+visible = false
+layout_mode = 2
+mouse_filter = 2
+texture = ExtResource("64_rur7c")
+
+[node name="Control" type="Control" parent="Stage/Credits" unique_id=1835435348]
+layout_mode = 2
+size_flags_vertical = 3
+
 [node name="ItemAmountUpdate" type="Timer" parent="Stage" unique_id=1953928024]
 wait_time = 0.2
 autostart = true