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.gd

@@ -0,0 +1,55 @@
+extends Control
+class_name InventorySlot
+
+signal clicked(item_id: String)
+signal right_clicked(item_id: String)
+signal hovered(item_id: String)
+signal unhovered(item_id: String)
+
+var item_id: String = ""
+var is_hovered: bool = false
+
+@onready var item_box: ColorRect = $ItemBox
+@onready var hover_highlight: ColorRect = $HoverHighlight
+
+func _ready() -> void:
+	hover_highlight.visible = false
+
+func set_item(item_def: ItemDefinition) -> void:
+	item_id = item_def.id
+	item_box.color = Color(1, 0.6, 0.2, 1)
+
+func set_item_color(color: Color) -> void:
+	item_box.color = color
+
+func set_hover(hovered: bool) -> void:
+	is_hovered = hovered
+	hover_highlight.visible = hovered
+	if hovered:
+		item_box.color = item_box.color.lightened(0.2)
+	else:
+		var def = InventoryManager.get_item_definition(item_id)
+		if def:
+			item_box.color = Color(1, 0.6, 0.2, 1)
+		else:
+			item_box.color = Color(0.8, 0.8, 0.8, 1)
+
+func _gui_input(event: InputEvent) -> void:
+	if event is InputEventMouseButton:
+		if event.button_index == 1 and event.pressed:
+			clicked.emit(item_id)
+		elif event.button_index == 2:
+			right_clicked.emit(item_id)
+
+func _input(event: InputEvent) -> void:
+	if event is InputEventMouseMotion:
+		var rect = get_global_rect()
+		var mouse_pos = get_global_mouse_position()
+		var was_hovered = is_hovered
+		is_hovered = rect.has_point(mouse_pos)
+		if is_hovered != was_hovered:
+			set_hover(is_hovered)
+			if is_hovered:
+				hovered.emit(item_id)
+			else:
+				unhovered.emit(item_id)