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_backpack/InventoryBackpack.gd

@@ -0,0 +1,233 @@
+extends Control
+class_name InventoryBackpack
+
+signal overlay_show_requested
+signal overlay_hide_requested
+signal item_selected(item_id: String)
+signal returning_to_idle
+signal skip_action_requested
+
+enum State { IDLE, OPEN, SELECTED, ACQUIRE, REMOVE }
+
+var _state: State = State.IDLE
+var _animating: bool = false
+var _active_tween: Tween = null
+var _floating_item_color: Color = Color(1, 1, 1, 0)
+
+@onready var backpack_icon: ColorRect = $BackpackIcon
+@onready var floating_item: ColorRect = $FloatingItem
+@onready var animation_player: AnimationPlayer = $AnimationPlayer
+
+func _ready() -> void:
+	floating_item.modulate = Color(1, 1, 1, 0)
+	floating_item.visible = false
+	InventoryManager.item_acquired.connect(_on_item_acquired)
+	InventoryManager.item_removed.connect(_on_item_removed)
+	InventoryManager.inventory_changed.connect(_on_inventory_changed)
+	_update_floating_item()
+
+func get_state() -> State:
+	return _state
+
+func is_busy() -> bool:
+	return _animating
+
+func _update_floating_item() -> void:
+	if InventoryManager.selected_item:
+		var def = InventoryManager.get_item_definition(InventoryManager.selected_item)
+		if def:
+			floating_item.visible = true
+			floating_item.modulate = Color(1, 0.6, 0.2, 1)
+		else:
+			floating_item.visible = true
+			floating_item.modulate = Color(0.8, 0.8, 0.8, 1)
+	else:
+		floating_item.visible = false
+
+func transition_to(new_state: State) -> void:
+	if _animating and new_state != _state:
+		_kill_tween()
+	match new_state:
+		State.IDLE:
+			_transition_to_idle()
+		State.OPEN:
+			_transition_to_open()
+		State.SELECTED:
+			_transition_to_selected()
+		State.ACQUIRE:
+			_transition_to_acquire()
+		State.REMOVE:
+			_transition_to_remove()
+
+func _transition_to_idle() -> void:
+	if _state == State.IDLE:
+		return
+	_set_state(State.IDLE)
+	overlay_hide_requested.emit()
+	backpack_icon.modulate = Color(1, 1, 1, 1)
+	floating_item.visible = false
+
+	var tween = create_tween().bind_node(self)
+	_active_tween = tween
+	_animating = true
+	tween.tween_property(backpack_icon, "rotation", 0.0, 0.35).set_trans(Tween.TRANS_LINEAR)
+	tween.tween_property(backpack_icon, "position", backpack_icon.position, 0.35).set_trans(Tween.TRANS_SINE_IN)
+	tween.tween_callback(_on_transition_complete).bind(State.IDLE)
+
+func _transition_to_open() -> void:
+	var can_open = _check_guards()
+	if not can_open:
+		if _state == State.IDLE:
+			skip_action_requested.emit()
+		return
+
+	_set_state(State.OPEN)
+
+	var tween = create_tween().bind_node(self)
+	_active_tween = tween
+	_animating = true
+	tween.tween_property(backpack_icon, "rotation", PI / 4, 0.35).set_trans(Tween.TRANS_LINEAR)
+	tween.tween_property(backpack_icon, "position", Vector2(backpack_icon.position.x + 20, backpack_icon.position.y + 20), 0.35).set_trans(Tween.TRANS_SINE_IN)
+	tween.tween_callback(_on_transition_complete).bind(State.OPEN)
+	tween.tween_callback(overlay_show_requested.emit)
+
+func _transition_to_selected() -> void:
+	if _state == State.SELECTED:
+		return
+
+	_set_state(State.SELECTED)
+	_update_floating_item()
+
+	var tween = create_tween().bind_node(self)
+	_active_tween = tween
+	_animating = true
+
+	floating_item.visible = true
+	floating_item.modulate = Color(1, 1, 1, 0)
+	tween.tween_property(floating_item, "modulate", Color(1, 1, 1, 1), 0.5).set_trans(Tween.TRANS_LINEAR)
+	tween.tween_callback(_on_transition_complete).bind(State.SELECTED)
+
+func _transition_to_acquire() -> void:
+	if _state == State.ACQUIRE or _state == State.REMOVE:
+		_kill_tween()
+
+	_set_state(State.ACQUIRE)
+
+	var tween = create_tween().bind_node(self)
+	_active_tween = tween
+	_animating = true
+
+	backpack_icon.modulate = Color(1, 1, 1, 1)
+	tween.tween_property(backpack_icon, "rotation", PI / 4, 0.35).set_trans(Tween.TRANS_LINEAR)
+	tween.tween_callback(_start_acquire_item_anim)
+
+func _start_acquire_item_anim() -> void:
+	var tween = create_tween().bind_node(self)
+	_active_tween = tween
+
+	floating_item.visible = true
+	floating_item.modulate = Color(1, 1, 1, 0)
+	floating_item.position = Vector2(backpack_icon.position.x - 20, backpack_icon.position.y - 40)
+
+	tween.tween_property(floating_item, "modulate", Color(1, 1, 1, 1), 0.5).set_trans(Tween.TRANS_LINEAR)
+	tween.tween_property(floating_item, "position", backpack_icon.position, 0.5).set_trans(Tween.TRANS_LINEAR)
+	tween.tween_callback(_on_acquire_complete)
+
+func _on_acquire_complete() -> void:
+	floating_item.visible = false
+	transition_to(State.IDLE)
+
+func _transition_to_remove() -> void:
+	if _state == State.ACQUIRE or _state == State.REMOVE:
+		_kill_tween()
+
+	_set_state(State.REMOVE)
+
+	var tween = create_tween().bind_node(self)
+	_active_tween = tween
+	_animating = true
+
+	backpack_icon.modulate = Color(1, 1, 1, 1)
+	tween.tween_property(backpack_icon, "rotation", PI / 4, 0.35).set_trans(Tween.TRANS_LINEAR)
+	tween.tween_callback(_start_remove_item_anim)
+
+func _start_remove_item_anim() -> void:
+	var tween = create_tween().bind_node(self)
+	_active_tween = tween
+
+	floating_item.visible = true
+	floating_item.modulate = Color(1, 1, 1, 1)
+	floating_item.position = backpack_icon.position
+
+	tween.tween_property(floating_item, "position", Vector2(backpack_icon.position.x - 20, backpack_icon.position.y - 80), 0.5).set_trans(Tween.TRANS_LINEAR)
+	tween.parallel().tween_property(floating_item, "modulate", Color(1, 1, 1, 0), 0.5).set_trans(Tween.TRANS_LINEAR)
+	tween.tween_callback(_on_remove_complete)
+
+func _on_remove_complete() -> void:
+	floating_item.visible = false
+	var was_selected = InventoryManager.selected_item != ""
+	transition_to(State.IDLE)
+	if was_selected:
+		InventoryManager.clear_selection()
+
+func _set_state(new_state: State) -> void:
+	_state = new_state
+
+func _check_guards() -> bool:
+	var main_game = get_node_or_null("/root/Node2D")
+	if not main_game:
+		return true
+
+	if main_game.is_script_running:
+		return false
+
+	var fade = get_node_or_null("/root/Node2D/SceneDisplay/Fade")
+	if fade and fade.modulate.a > 0.5:
+		return false
+
+	return true
+
+func _kill_tween() -> void:
+	if _active_tween:
+		_active_tween.kill()
+		_active_tween = null
+	_animating = false
+
+func _on_transition_complete(completed_state: State) -> void:
+	_animating = false
+	_active_tween = null
+	if completed_state == State.OPEN:
+		pass
+	elif completed_state == State.IDLE:
+		returning_to_idle.emit()
+
+func _on_item_acquired(item_id: String) -> void:
+	if _state == State.IDLE:
+		transition_to(State.ACQUIRE)
+
+func _on_item_removed(item_id: String) -> void:
+	if _state == State.IDLE:
+		if InventoryManager.selected_item == item_id:
+			InventoryManager.clear_selection()
+			_update_floating_item()
+			transition_to(State.REMOVE)
+		else:
+			transition_to(State.REMOVE)
+	elif _state == State.SELECTED and InventoryManager.selected_item == item_id:
+		InventoryManager.clear_selection()
+		_update_floating_item()
+		transition_to(State.REMOVE)
+
+func _on_inventory_changed() -> void:
+	_update_floating_item()
+
+func _gui_input(event: InputEvent) -> void:
+	if event is InputEventMouseButton and event.pressed and event.button_index == 1:
+		if _state == State.IDLE:
+			transition_to(State.OPEN)
+		elif _state == State.OPEN:
+			transition_to(State.IDLE)
+
+func _notification(what: int) -> void:
+	if what == NOTIFICATION_PREDELETE:
+		_kill_tween()

inventory/inventory_backpack/InventoryBackpack.gd.uid

@@ -0,0 +1 @@
+uid://2x3g0ethsdcgo

inventory/inventory_backpack/InventoryBackpack.tscn

@@ -0,0 +1,31 @@
+[gd_scene format=3 uid="uid://1406xmcnkygw0"]
+
+[ext_resource type="Script" uid="uid://2x3g0ethsdcgo" path="res://inventory/inventory_backpack/InventoryBackpack.gd" id="1"]
+
+[node name="InventoryBackpack" type="Control" unique_id=1000000001]
+layout_mode = 3
+anchors_preset = 2
+anchor_right = 1.0
+offset_top = 10.0
+offset_right = -10.0
+offset_bottom = 70.0
+script = ExtResource("1")
+
+[node name="BackpackIcon" type="ColorRect" parent="." unique_id=1000000002]
+layout_mode = 0
+offset_left = 0.0
+offset_top = 0.0
+offset_right = 60.0
+offset_bottom = 60.0
+color = Color(0.4, 0.6, 0.9, 1)
+
+[node name="FloatingItem" type="ColorRect" parent="." unique_id=1000000003]
+layout_mode = 0
+offset_left = 20.0
+offset_top = -30.0
+offset_right = 50.0
+offset_bottom = 0.0
+color = Color(1, 0.6, 0.2, 1)
+visible = false
+
+[node name="AnimationPlayer" type="AnimationPlayer" parent="." unique_id=1000000004]

inventory/inventory_backpack/InventoryBackpack.tscn.uid

@@ -0,0 +1 @@
+uid://1406xmcnkygw0

inventory/inventory_overlay/InventoryOverlay.gd

@@ -0,0 +1,215 @@
+extends Control
+class_name InventoryOverlay
+
+signal close_requested
+signal item_confirmed(item_id: String)
+signal combine_requested(item_a_id: String, item_b_id: String)
+signal inspect_requested(item_id: String)
+
+var input_active: bool = false
+var _is_visible: bool = false
+var _selected_slot: InventorySlot = null
+var _drag_start_time: float = 0.0
+var _is_dragging: bool = false
+var _dragged_item: Control = null
+var _hovered_slot: InventorySlot = null
+var _fade_tween: Tween = null
+
+const FADE_DURATION: float = 0.2
+const LONG_PRESS_THRESHOLD: float = 0.5
+const SLOTS_PER_ROW: int = 8
+const SLOT_SIZE: Vector2i = Vector2i(64, 64)
+
+@onready var background: ColorRect = $Background
+@onready var panel: Control = $InventoryPanel
+@onready var frame: ColorRect = $InventoryPanel/Frame
+@onready var grid: GridContainer = $InventoryPanel/ItemGrid
+@onready var hover_label: Label = $InventoryPanel/HoverLabel
+
+func _ready() -> void:
+	hide()
+	background.mouse_filter = Control.MOUSE_FILTER_STOP
+	InventoryManager.inventory_changed.connect(_refresh_grid)
+	InventoryManager.combination_attempted.connect(_on_combination_attempted)
+
+func show_overlay() -> void:
+	if _fade_tween:
+		_fade_tween.kill()
+		_fade_tween = null
+
+	_is_visible = true
+	show()
+	modulate = Color(1, 1, 1, 0)
+	input_active = false
+
+	var tween = create_tween().bind_node(self)
+	_fade_tween = tween
+	tween.tween_property(self, "modulate", Color(1, 1, 1, 1), FADE_DURATION)
+	tween.tween_callback(_on_fade_in_complete)
+
+func hide_overlay() -> void:
+	if _fade_tween:
+		_fade_tween.kill()
+		_fade_tween = null
+
+	input_active = false
+	_clear_selection()
+
+	var tween = create_tween().bind_node(self)
+	_fade_tween = tween
+	tween.tween_property(self, "modulate", Color(1, 1, 1, 0), FADE_DURATION)
+	tween.tween_callback(_on_fade_out_complete)
+
+func _on_fade_in_complete() -> void:
+	input_active = true
+
+func _on_fade_out_complete() -> void:
+	_is_visible = false
+	hide()
+	input_active = false
+
+func is_active() -> bool:
+	return _is_visible and input_active
+
+func _refresh_grid() -> void:
+	for child in grid.get_children():
+		child.queue_free()
+
+	for item_id in InventoryManager.inventory:
+		var def = InventoryManager.get_item_definition(item_id)
+		if not def:
+			def = ItemDefinition.new()
+			def.id = item_id
+			def.name = item_id
+
+		var slot_scene = load("res://inventory/inventory_overlay/InventorySlot.tscn")
+		var slot: InventorySlot = slot_scene.instantiate()
+		slot.set_item(def)
+		slot.clicked.connect(_on_slot_clicked)
+		slot.right_clicked.connect(_on_slot_right_clicked)
+		slot.hovered.connect(_on_slot_hovered)
+		slot.unhovered.connect(_on_slot_unhovered)
+		grid.add_child(slot)
+
+	grid.columns = SLOTS_PER_ROW
+
+func _on_slot_clicked(item_id: String) -> void:
+	if not input_active:
+		return
+
+	for child in grid.get_children():
+		if child is InventorySlot and child.item_id == item_id:
+			if _selected_slot == null:
+				_selected_slot = child
+				_drag_start_time = Time.get_ticks_msec() / 1000.0
+				_is_dragging = true
+				_create_drag_preview(child)
+			elif _selected_slot.item_id == item_id:
+				_handle_release_same_item()
+			else:
+				combine_requested.emit(_selected_slot.item_id, item_id)
+				_clear_selection()
+			break
+
+func _on_slot_right_clicked(item_id: String) -> void:
+	if not input_active:
+		return
+	inspect_requested.emit(item_id)
+	hide_overlay()
+
+func _on_slot_hovered(item_id: String) -> void:
+	if not input_active:
+		return
+	_hovered_slot = null
+	for child in grid.get_children():
+		if child is InventorySlot and child.item_id == item_id:
+			_hovered_slot = child
+			break
+	_update_hover_label()
+
+func _on_slot_unhovered(item_id: String) -> void:
+	if _hovered_slot and _hovered_slot.item_id == item_id:
+		_hovered_slot = null
+	_update_hover_label()
+
+func _create_drag_preview(slot: InventorySlot) -> void:
+	var preview = ColorRect.new()
+	preview.color = Color(1, 0.6, 0.2, 0.7)
+	preview.size = Vector2(48, 48)
+	preview.position = slot.global_position - global_position
+	panel.add_child(preview)
+	_dragged_item = preview
+
+func _handle_release_same_item() -> void:
+	var elapsed = Time.get_ticks_msec() / 1000.0 - _drag_start_time
+	if elapsed <= LONG_PRESS_THRESHOLD:
+		item_confirmed.emit(_selected_slot.item_id)
+		hide_overlay()
+	else:
+		_clear_selection()
+
+func _clear_selection() -> void:
+	_selected_slot = null
+	_is_dragging = false
+	if _dragged_item:
+		_dragged_item.queue_free()
+		_dragged_item = null
+	_update_hover_label()
+
+func _update_hover_label() -> void:
+	if _selected_slot and _hovered_slot and _selected_slot.item_id != _hovered_slot.item_id:
+		var def_a = InventoryManager.get_item_definition(_selected_slot.item_id)
+		var def_b = InventoryManager.get_item_definition(_hovered_slot.item_id)
+		var name_a = _selected_slot.item_id
+		var name_b = _hovered_slot.item_id
+		if def_a:
+			name_a = def_a.name
+		if def_b:
+			name_b = def_b.name
+		hover_label.text = "Use %s with %s" % [name_a, name_b]
+	elif _hovered_slot:
+		var def = InventoryManager.get_item_definition(_hovered_slot.item_id)
+		if def:
+			hover_label.text = def.name
+		else:
+			hover_label.text = _hovered_slot.item_id
+	else:
+		hover_label.text = ""
+
+func _gui_input(event: InputEvent) -> void:
+	if not _is_visible:
+		return
+
+	if event is InputEventMouseButton:
+		if event.button_index == 1 and event.pressed:
+			if not _is_dragging:
+				pass
+		elif event.button_index == 1 and not event.pressed:
+			if _is_dragging:
+				if _hovered_slot == null:
+					_clear_selection()
+					hide_overlay()
+					close_requested.emit()
+				elif _hovered_slot == _selected_slot:
+					_handle_release_same_item()
+				else:
+					combine_requested.emit(_selected_slot.item_id, _hovered_slot.item_id)
+					_clear_selection()
+			return
+
+	if event is InputEventMouseMotion and _is_dragging and _dragged_item:
+		_dragged_item.position = get_local_mouse_position() - Vector2(24, 24)
+		var panel_rect = panel.get_global_rect()
+		if not panel_rect.has_point(get_global_mouse_position()):
+			_clear_selection()
+			hide_overlay()
+			close_requested.emit()
+
+func _on_combination_attempted(item_a_id: String, item_b_id: String) -> void:
+	pass
+
+func _notification(what: int) -> void:
+	if what == NOTIFICATION_PREDELETE:
+		if _fade_tween:
+			_fade_tween.kill()
+			_fade_tween = null

inventory/inventory_overlay/InventoryOverlay.gd.uid

@@ -0,0 +1 @@
+uid://3mkdj9s1oe1jz

inventory/inventory_overlay/InventoryOverlay.tscn

@@ -0,0 +1,63 @@
+[gd_scene format=3 uid="uid://1p46uzngsih9o"]
+
+[ext_resource type="Script" uid="uid://3mkdj9s1oe1jz" path="res://inventory/inventory_overlay/InventoryOverlay.gd" id="1"]
+
+[sub_resource type="LabelSettings" id="LabelSettings_inv"]
+font_size = 20
+outline_size = 3
+outline_color = Color(0, 0, 0, 1)
+
+[node name="InventoryOverlay" type="Control" unique_id=3000000001]
+layout_mode = 3
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+script = ExtResource("1")
+
+[node name="Background" type="ColorRect" parent="." unique_id=3000000002]
+layout_mode = 1
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+color = Color(0, 0, 0, 0.7)
+
+[node name="InventoryPanel" type="Control" parent="." unique_id=3000000003]
+layout_mode = 1
+anchors_preset = 7
+anchor_left = 0.5
+anchor_top = 0.5
+anchor_right = 0.5
+anchor_bottom = 0.5
+offset_left = -350.0
+offset_top = -150.0
+offset_right = 350.0
+offset_bottom = 150.0
+grow_horizontal = 2
+grow_vertical = 2
+
+[node name="Frame" type="ColorRect" parent="InventoryPanel" unique_id=3000000004]
+layout_mode = 1
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+color = Color(0.15, 0.15, 0.2, 1)
+
+[node name="ItemGrid" type="GridContainer" parent="InventoryPanel" unique_id=3000000005]
+layout_mode = 1
+anchors_preset = 0
+offset_left = 10.0
+offset_top = 10.0
+offset_right = -10.0
+offset_bottom = -50.0
+
+[node name="HoverLabel" type="Label" parent="InventoryPanel" unique_id=3000000006]
+layout_mode = 1
+anchors_preset = 8
+anchor_left = 0.0
+anchor_top = 1.0
+anchor_right = 1.0
+anchor_bottom = 1.0
+offset_bottom = -10.0
+grow_horizontal = 2
+horizontal_alignment = 1
+label_settings = SubResource("LabelSettings_inv")

inventory/inventory_overlay/InventoryOverlay.tscn.uid

@@ -0,0 +1 @@
+uid://1p46uzngsih9o

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)

inventory/inventory_overlay/InventorySlot.gd.uid

@@ -0,0 +1 @@
+uid://oegm753jbl9m

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

inventory/inventory_overlay/InventorySlot.tscn.uid

@@ -0,0 +1 @@
+uid://1esl88fgtd2p6