feat: implement InventoryBackpack FSM and InventoryOverlay

- InventoryBackpack: Control-based FSM with IDLE/OPEN/SELECTED/ACQUIRE/REMOVE
  states, Tween-based animations, guard condition checks, signal connections
  to InventoryManager for item_acquired/item_removed reactions
- InventoryOverlay: Full-screen overlay with fade-in/out, item grid via
  GridContainer, drag-and-drop item selection, combination via drag-to-slot,
  hover labels, right-click inspect
- InventorySlot: Individual slot with colored box placeholder, hover highlight,
  click/right-click/hover signals

inventory/inventory_overlay/InventorySlot.tscn

@@ -0,0 +1,26 @@
+[gd_scene format=3 uid="uid://1esl88fgtd2p6"]
+
+[ext_resource type="Script" uid="uid://oegm753jbl9m" path="res://inventory/inventory_overlay/InventorySlot.gd" id="1"]
+
+[node name="InventorySlot" type="Control" unique_id=2000000001]
+custom_minimum_size = Vector2i(64, 64)
+script = ExtResource("1")
+
+[node name="ItemBox" type="ColorRect" parent="." unique_id=2000000002]
+layout_mode = 1
+anchors_preset = 8
+anchor_left = 0.062
+anchor_top = 0.062
+anchor_right = 0.938
+anchor_bottom = 0.938
+color = Color(1, 0.6, 0.2, 1)
+
+[node name="HoverHighlight" type="ColorRect" parent="." unique_id=2000000003]
+layout_mode = 1
+anchors_preset = 8
+anchor_left = 0.0
+anchor_top = 0.0
+anchor_right = 1.0
+anchor_bottom = 1.0
+color = Color(1, 1, 1, 0.3)
+visible = false