chore: enable ETC2/ASTC texture compression for Android export

- Add RectangleShape2D to Character and InteractiveObject collision areas
- Fix HUD button signal connections in _ready()
- Fix character_placed signal emitting global_position
- Extract DEFAULT_DRAG_RADIUS constant in DragDropComponent
- Type Variant on JSON parsed variable in SaveManager
- Extract music symbol constants in HUD
- Refactor duplicated drag input code in InputManager

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.editorconfig

@@ -0,0 +1,23 @@
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.gd]
+indent_style = tab
+indent_size = 4
+
+[*.{tscn,tres,godot,cfg}]
+indent_style = tab
+
+[*.md]
+trim_trailing_whitespace = false
+indent_style = space
+indent_size = 2
+
+[*.json]
+indent_style = space
+indent_size = 2

docs/smoke-tests.md

@@ -0,0 +1,58 @@
+# Smoke-Test-Checkliste
+
+> Smoke Tests werden nach jedem Sprint manuell auf einem echten Android-Tablet durchgeführt.
+> Ziel ist kein vollständiger Test, sondern ein schnelles "Geht es grundlegend?" vor dem Zeigen an die Kinder.
+> Neue Sprint-Abschnitte werden jeweils zu Beginn des Sprints ergänzt.
+
+---
+
+## Basis-Tests (jeder Sprint)
+
+Diese Tests werden bei jedem Sprint-Abschluss durchgeführt, unabhängig vom Inhalt.
+
+- [ ] App startet ohne Absturz
+- [ ] Kein Freeze oder schwarzer Screen in den ersten 5 Sekunden
+- [ ] Musik spielt beim Start (ca. 60% Lautstärke)
+- [ ] HUD: Back-Button sichtbar und funktionsfähig
+- [ ] HUD: Musik-Toggle funktioniert (an/aus)
+- [ ] Performance: Keine sichtbaren Ruckler beim Scrollen/Navigieren
+- [ ] Touch-Targets: Alle interaktiven Elemente mit Kinderfingern erreichbar (min. 48dp)
+- [ ] Offline: Spiel funktioniert vollständig ohne WLAN/Mobilfunk
+- [ ] Kein Netzwerk-Request im Hintergrund (DevTools / Android Profiler prüfen)
+
+---
+
+## Sprint 1 — Proof of Concept
+
+Ziel: Erster spielbarer Raum (Empfang/Reception), Drag & Drop einer Figur, ein interaktives Objekt.
+
+- [ ] Empfangs-Raum lädt ohne Fehler
+- [ ] Figur (Häschen oder Kätzchen) ist auf dem Bildschirm sichtbar
+- [ ] Figur lässt sich per Touch aufheben (Drag-Start)
+- [ ] Figur lässt sich an beliebiger Stelle loslassen (Drop)
+- [ ] Figur kehrt nicht automatisch zur Startposition zurück
+- [ ] Interaktives Objekt (Blume) reagiert auf Touch (Animation oder Sound)
+- [ ] Position der Figur bleibt nach App-Neustart erhalten (Save/Load)
+- [ ] Keine GDScript-Fehler in der Godot-Konsole
+
+---
+
+## Sprint 2 — (wird ergänzt)
+
+---
+
+## Sprint 3 — (wird ergänzt)
+
+---
+
+## Sprint 4 — (wird ergänzt)
+
+---
+
+## Sprint 5 — (wird ergänzt)
+
+---
+
+## Notizen
+
+_Auffälligkeiten, Geräte-spezifische Bugs oder UAT-Feedback der Kinder hier festhalten._

project.godot

@@ -0,0 +1,42 @@
+; Engine configuration file.
+; It's best edited using the editor UI and not directly,
+; since the parameters that go here are not all obvious.
+;
+; Format:
+;   [section] ; section goes between []
+;   param=value ; assign values to parameters
+
+config_version=5
+
+[application]
+
+config/name="Cozypaw Hospital"
+config/description="Werbefrei. Offline. Für Kinder."
+config/version="0.1.0"
+run/main_scene="res://scenes/main/Main.tscn"
+config/features=PackedStringArray("4.6", "Mobile")
+boot_splash/show_image=false
+
+[autoload]
+
+GameState="*res://scripts/autoload/GameState.gd"
+SaveManager="*res://scripts/autoload/SaveManager.gd"
+AudioManager="*res://scripts/autoload/AudioManager.gd"
+InputManager="*res://scripts/autoload/InputManager.gd"
+
+[display]
+
+window/size/viewport_width=1280
+window/size/viewport_height=720
+window/stretch/mode="canvas_items"
+window/stretch/aspect="expand"
+
+[input_devices]
+
+pointing/emulate_touch_from_mouse=true
+
+[rendering]
+
+renderer/rendering_method="gl_compatibility"
+renderer/rendering_method.mobile="gl_compatibility"
+textures/vram_compression/import_etc2_astc=true

scenes/characters/Character.tscn

@@ -0,0 +1,51 @@
+[gd_scene load_steps=4 format=3 uid="uid://cozypaw_char"]
+
+[ext_resource type="Script" path="res://scripts/characters/character.gd" id="1_char"]
+[ext_resource type="Script" path="res://scripts/systems/drag_drop_component.gd" id="2_drag"]
+
+[sub_resource type="RectangleShape2D" id="RectangleShape2D_char"]
+size = Vector2(64, 80)
+
+[node name="Character" type="Node2D"]
+script = ExtResource("1_char")
+character_id = "bunny_01"
+display_name = "Bunny"
+
+[node name="Visual" type="ColorRect" parent="."]
+color = Color(0.95, 0.85, 0.9, 1)
+size = Vector2(64, 80)
+position = Vector2(-32, -80)
+
+[node name="Eyes" type="Node2D" parent="Visual"]
+
+[node name="EyeLeft" type="ColorRect" parent="Visual/Eyes"]
+color = Color(0.1, 0.1, 0.1, 1)
+size = Vector2(8, 8)
+position = Vector2(14, 20)
+
+[node name="EyeRight" type="ColorRect" parent="Visual/Eyes"]
+color = Color(0.1, 0.1, 0.1, 1)
+size = Vector2(8, 8)
+position = Vector2(42, 20)
+
+[node name="Ears" type="Node2D" parent="."]
+
+[node name="EarLeft" type="ColorRect" parent="Ears"]
+color = Color(0.95, 0.85, 0.9, 1)
+size = Vector2(18, 36)
+position = Vector2(-22, -114)
+
+[node name="EarRight" type="ColorRect" parent="Ears"]
+color = Color(0.95, 0.85, 0.9, 1)
+size = Vector2(18, 36)
+position = Vector2(4, -114)
+
+[node name="DragDropComponent" type="Node" parent="."]
+script = ExtResource("2_drag")
+
+[node name="CollisionArea" type="Area2D" parent="."]
+input_pickable = true
+
+[node name="CollisionShape" type="CollisionShape2D" parent="CollisionArea"]
+shape = SubResource("RectangleShape2D_char")
+position = Vector2(0, -40)

scenes/main/Main.tscn

@@ -0,0 +1,23 @@
+[gd_scene load_steps=4 format=3 uid="uid://cozypaw_main"]
+
+[ext_resource type="PackedScene" path="res://scenes/rooms/floor0/Reception.tscn" id="1_reception"]
+[ext_resource type="PackedScene" path="res://scenes/characters/Character.tscn" id="2_char"]
+[ext_resource type="PackedScene" path="res://scenes/ui/HUD.tscn" id="3_hud"]
+
+[node name="Main" type="Node2D"]
+
+[node name="Hospital" type="Node2D" parent="."]
+
+[node name="Floor0" type="Node2D" parent="Hospital"]
+
+[node name="Reception" parent="Hospital/Floor0" instance=ExtResource("1_reception")]
+position = Vector2(0, 0)
+
+[node name="Characters" type="Node2D" parent="."]
+
+[node name="Bunny1" parent="Characters" instance=ExtResource("2_char")]
+position = Vector2(300, 400)
+
+[node name="UI" type="CanvasLayer" parent="."]
+
+[node name="HUD" parent="UI" instance=ExtResource("3_hud")]

scenes/objects/InteractiveObject.tscn

@@ -0,0 +1,32 @@
+[gd_scene load_steps=4 format=3 uid="uid://cozypaw_iobj"]
+
+[ext_resource type="Script" path="res://scripts/objects/interactive_object.gd" id="1_iobj"]
+[ext_resource type="Script" path="res://scripts/systems/drag_drop_component.gd" id="2_drag"]
+
+[sub_resource type="RectangleShape2D" id="RectangleShape2D_obj"]
+size = Vector2(48, 48)
+
+[node name="InteractiveObject" type="Node2D"]
+script = ExtResource("1_iobj")
+object_id = "flower_01"
+
+[node name="Visual" type="Node2D" parent="."]
+
+[node name="Stem" type="ColorRect" parent="Visual"]
+color = Color(0.2, 0.7, 0.2, 1)
+size = Vector2(8, 40)
+position = Vector2(-4, -40)
+
+[node name="Bloom" type="ColorRect" parent="Visual"]
+color = Color(1.0, 0.4, 0.6, 1)
+size = Vector2(32, 32)
+position = Vector2(-16, -72)
+
+[node name="DragDropComponent" type="Node" parent="."]
+script = ExtResource("2_drag")
+
+[node name="CollisionArea" type="Area2D" parent="."]
+input_pickable = true
+
+[node name="CollisionShape" type="CollisionShape2D" parent="CollisionArea"]
+shape = SubResource("RectangleShape2D_obj")

scenes/rooms/floor0/Reception.tscn

@@ -0,0 +1,41 @@
+[gd_scene load_steps=2 format=3 uid="uid://cozypaw_reception"]
+
+[ext_resource type="PackedScene" path="res://scenes/objects/InteractiveObject.tscn" id="1_iobj"]
+
+[node name="Reception" type="Node2D"]
+
+[node name="Background" type="ColorRect" parent="."]
+color = Color(0.78, 0.94, 0.80, 1)
+size = Vector2(1280, 720)
+position = Vector2(0, 0)
+
+[node name="Counter" type="ColorRect" parent="."]
+color = Color(0.55, 0.35, 0.18, 1)
+size = Vector2(300, 80)
+position = Vector2(490, 610)
+
+[node name="CounterTop" type="ColorRect" parent="."]
+color = Color(0.70, 0.50, 0.28, 1)
+size = Vector2(300, 12)
+position = Vector2(490, 598)
+
+[node name="Floor" type="ColorRect" parent="."]
+color = Color(0.88, 0.80, 0.68, 1)
+size = Vector2(1280, 100)
+position = Vector2(0, 620)
+
+[node name="WallLeft" type="ColorRect" parent="."]
+color = Color(0.92, 0.88, 0.82, 1)
+size = Vector2(40, 620)
+position = Vector2(0, 0)
+
+[node name="WallRight" type="ColorRect" parent="."]
+color = Color(0.92, 0.88, 0.82, 1)
+size = Vector2(40, 620)
+position = Vector2(1240, 0)
+
+[node name="CharacterSpawn" type="Marker2D" parent="."]
+position = Vector2(300, 520)
+
+[node name="Flower" parent="." instance=ExtResource("1_iobj")]
+position = Vector2(200, 560)

scenes/ui/HUD.tscn

@@ -0,0 +1,35 @@
+[gd_scene load_steps=2 format=3 uid="uid://cozypaw_hud"]
+
+[ext_resource type="Script" path="res://scripts/systems/hud.gd" id="1_hud"]
+
+[node name="HUD" type="CanvasLayer"]
+script = ExtResource("1_hud")
+
+[node name="BackButton" type="Button" parent="."]
+anchors_preset = 0
+anchor_left = 0.0
+anchor_top = 0.0
+anchor_right = 0.0
+anchor_bottom = 0.0
+offset_left = 16.0
+offset_top = 16.0
+offset_right = 80.0
+offset_bottom = 80.0
+text = "←"
+flat = false
+
+[node name="MusicToggle" type="Button" parent="."]
+anchors_preset = 1
+anchor_left = 1.0
+anchor_top = 0.0
+anchor_right = 1.0
+anchor_bottom = 0.0
+offset_left = -80.0
+offset_top = 16.0
+offset_right = -16.0
+offset_bottom = 80.0
+text = "♪"
+flat = false
+
+[connection signal="pressed" from="BackButton" to="." method="_on_back_button_pressed"]
+[connection signal="pressed" from="MusicToggle" to="." method="_on_music_toggle_pressed"]

scripts/autoload/AudioManager.gd

@@ -0,0 +1,63 @@
+## AudioManager — music playback with cross-fade, SFX playback, volume control.
+extends Node
+
+const DEFAULT_MUSIC_VOLUME: float = 0.6
+const CROSSFADE_DURATION: float = 1.0
+
+var _music_player_a: AudioStreamPlayer
+var _music_player_b: AudioStreamPlayer
+var _active_player: AudioStreamPlayer
+var _sfx_player: AudioStreamPlayer
+var _music_volume: float = DEFAULT_MUSIC_VOLUME
+var _sfx_volume: float = 1.0
+var _is_fading: bool = false
+
+
+func _ready() -> void:
+	_music_player_a = AudioStreamPlayer.new()
+	_music_player_b = AudioStreamPlayer.new()
+	_sfx_player = AudioStreamPlayer.new()
+	add_child(_music_player_a)
+	add_child(_music_player_b)
+	add_child(_sfx_player)
+	_active_player = _music_player_a
+	_apply_music_volume()
+
+
+func play_music(stream: AudioStream) -> void:
+	if _active_player.stream == stream and _active_player.playing:
+		return
+	var next_player: AudioStreamPlayer = _music_player_b if _active_player == _music_player_a else _music_player_a
+	next_player.stream = stream
+	next_player.volume_db = linear_to_db(0.0)
+	next_player.play()
+	var tween: Tween = create_tween()
+	tween.set_parallel(true)
+	tween.tween_property(_active_player, "volume_db", linear_to_db(0.0), CROSSFADE_DURATION)
+	tween.tween_property(next_player, "volume_db", linear_to_db(_music_volume), CROSSFADE_DURATION)
+	var prev_player: AudioStreamPlayer = _active_player
+	tween.tween_callback(prev_player.stop).set_delay(CROSSFADE_DURATION)
+	_active_player = next_player
+
+
+func play_sfx(stream: AudioStream) -> void:
+	_sfx_player.stream = stream
+	_sfx_player.volume_db = linear_to_db(_sfx_volume)
+	_sfx_player.play()
+
+
+func set_music_volume(value: float) -> void:
+	_music_volume = clampf(value, 0.0, 1.0)
+	_apply_music_volume()
+
+
+func set_sfx_volume(value: float) -> void:
+	_sfx_volume = clampf(value, 0.0, 1.0)
+
+
+func get_music_volume() -> float:
+	return _music_volume
+
+
+func _apply_music_volume() -> void:
+	_active_player.volume_db = linear_to_db(_music_volume)

scripts/autoload/GameState.gd

@@ -0,0 +1,45 @@
+## GameState — global game state: character positions, object states, current room.
+extends Node
+
+signal state_changed
+signal character_moved(character_id: String, position: Vector2)
+
+var _character_positions: Dictionary = {}
+var _object_states: Dictionary = {}
+var current_room: String = "reception"
+
+
+func get_character_position(id: String) -> Vector2:
+	return _character_positions.get(id, Vector2.ZERO)
+
+
+func set_character_position(id: String, pos: Vector2) -> void:
+	_character_positions[id] = pos
+	character_moved.emit(id, pos)
+	state_changed.emit()
+
+
+func get_object_state(id: String) -> String:
+	return _object_states.get(id, "idle")
+
+
+func set_object_state(id: String, state: String) -> void:
+	_object_states[id] = state
+	state_changed.emit()
+
+
+func get_save_data() -> Dictionary:
+	return {
+		"character_positions": _character_positions,
+		"object_states": _object_states,
+		"current_room": current_room,
+	}
+
+
+func apply_save_data(data: Dictionary) -> void:
+	if data.has("character_positions"):
+		_character_positions = data["character_positions"]
+	if data.has("object_states"):
+		_object_states = data["object_states"]
+	if data.has("current_room"):
+		current_room = data["current_room"]

scripts/autoload/InputManager.gd

@@ -0,0 +1,40 @@
+## InputManager — abstracts touch and mouse input into unified drag signals.
+extends Node
+
+signal drag_started(position: Vector2)
+signal drag_moved(position: Vector2)
+signal drag_ended(position: Vector2)
+
+var _is_dragging: bool = false
+
+
+func _input(event: InputEvent) -> void:
+	if event is InputEventScreenTouch:
+		if event.pressed:
+			_emit_drag_started(event.position)
+		else:
+			_emit_drag_ended(event.position)
+	elif event is InputEventScreenDrag:
+		if _is_dragging:
+			_emit_drag_moved(event.position)
+	elif event is InputEventMouseButton and event.button_index == MOUSE_BUTTON_LEFT:
+		if event.pressed:
+			_emit_drag_started(event.position)
+		else:
+			_emit_drag_ended(event.position)
+	elif event is InputEventMouseMotion and _is_dragging:
+		_emit_drag_moved(event.position)
+
+
+func _emit_drag_started(pos: Vector2) -> void:
+	_is_dragging = true
+	drag_started.emit(pos)
+
+
+func _emit_drag_moved(pos: Vector2) -> void:
+	drag_moved.emit(pos)
+
+
+func _emit_drag_ended(pos: Vector2) -> void:
+	_is_dragging = false
+	drag_ended.emit(pos)

scripts/autoload/SaveManager.gd

@@ -0,0 +1,40 @@
+## SaveManager — persists game state as JSON to user://savegame.json, auto-saves on state_changed.
+extends Node
+
+const SAVE_PATH: String = "user://savegame.json"
+
+
+func _ready() -> void:
+	GameState.state_changed.connect(_on_state_changed)
+
+
+func save_game() -> void:
+	var data: Dictionary = GameState.get_save_data()
+	var file: FileAccess = FileAccess.open(SAVE_PATH, FileAccess.WRITE)
+	if file == null:
+		return
+	file.store_string(JSON.stringify(data))
+	file.close()
+
+
+func load_game() -> void:
+	if not FileAccess.file_exists(SAVE_PATH):
+		return
+	var file: FileAccess = FileAccess.open(SAVE_PATH, FileAccess.READ)
+	if file == null:
+		return
+	var raw: String = file.get_as_text()
+	file.close()
+	var parsed: Variant = JSON.parse_string(raw)
+	if parsed is Dictionary:
+		GameState.apply_save_data(parsed)
+
+
+func reset_game() -> void:
+	if FileAccess.file_exists(SAVE_PATH):
+		DirAccess.remove_absolute(SAVE_PATH)
+	GameState.apply_save_data({})
+
+
+func _on_state_changed() -> void:
+	save_game()

scripts/characters/character.gd

@@ -0,0 +1,28 @@
+## Character — base class for all playable figures (bunny, cat, etc.).
+class_name Character extends Node2D
+
+signal character_picked_up(character: Character)
+signal character_placed(character: Character, position: Vector2)
+
+@export var character_id: String = ""
+@export var display_name: String = ""
+
+var _is_held: bool = false
+
+
+func _ready() -> void:
+	var drag: DragDropComponent = get_node_or_null("DragDropComponent") as DragDropComponent
+	if drag != null:
+		drag.drag_picked_up.connect(_on_drag_picked_up)
+		drag.drag_released.connect(_on_drag_released)
+
+
+func _on_drag_picked_up(_pos: Vector2) -> void:
+	_is_held = true
+	character_picked_up.emit(self)
+
+
+func _on_drag_released(pos: Vector2) -> void:
+	_is_held = false
+	GameState.set_character_position(character_id, global_position)
+	character_placed.emit(self, global_position)

scripts/objects/interactive_object.gd

@@ -0,0 +1,57 @@
+## InteractiveObject — base class for all interactive room objects (flowers, equipment, etc.).
+class_name InteractiveObject extends Node2D
+
+signal object_interacted(object: InteractiveObject)
+
+enum State { IDLE, ACTIVE, RETURNING }
+
+@export var object_id: String = ""
+
+var _current_state: State = State.IDLE
+
+
+func _ready() -> void:
+	var drag: DragDropComponent = get_node_or_null("DragDropComponent") as DragDropComponent
+	if drag != null:
+		drag.drag_picked_up.connect(_on_drag_picked_up)
+		drag.drag_released.connect(_on_drag_released)
+	var area: Area2D = get_node_or_null("CollisionArea") as Area2D
+	if area != null:
+		area.input_event.connect(_on_area_input_event)
+
+
+func _on_drag_picked_up(_pos: Vector2) -> void:
+	_set_state(State.ACTIVE)
+	object_interacted.emit(self)
+
+
+func _on_drag_released(_pos: Vector2) -> void:
+	_set_state(State.RETURNING)
+	GameState.set_object_state(object_id, "idle")
+	_play_bounce_animation()
+
+
+func _on_area_input_event(_viewport: Node, event: InputEvent, _shape_idx: int) -> void:
+	if event is InputEventMouseButton and event.pressed and event.button_index == MOUSE_BUTTON_LEFT:
+		_trigger_interaction()
+	elif event is InputEventScreenTouch and event.pressed:
+		_trigger_interaction()
+
+
+func _trigger_interaction() -> void:
+	_set_state(State.ACTIVE)
+	object_interacted.emit(self)
+	GameState.set_object_state(object_id, "active")
+	_play_bounce_animation()
+
+
+func _set_state(new_state: State) -> void:
+	_current_state = new_state
+
+
+func _play_bounce_animation() -> void:
+	var tween: Tween = create_tween()
+	tween.tween_property(self, "scale", Vector2(1.2, 1.2), 0.1)
+	tween.tween_property(self, "scale", Vector2(0.9, 0.9), 0.1)
+	tween.tween_property(self, "scale", Vector2(1.0, 1.0), 0.1)
+	tween.tween_callback(func() -> void: _set_state(State.IDLE))

scripts/systems/drag_drop_component.gd

@@ -0,0 +1,81 @@
+## DragDropComponent — reusable drag-and-drop node; attach to any Character or InteractiveObject.
+class_name DragDropComponent extends Node
+
+signal drag_picked_up(global_position: Vector2)
+signal drag_released(global_position: Vector2)
+
+const DRAG_Z_INDEX: int = 10
+const DRAG_SCALE: float = 1.1
+const DEFAULT_DRAG_RADIUS: float = 64.0
+
+@export var drag_target: Node2D
+
+var _is_dragging: bool = false
+var _drag_offset: Vector2 = Vector2.ZERO
+var _original_z_index: int = 0
+var _original_scale: Vector2 = Vector2.ONE
+
+
+func _ready() -> void:
+	if drag_target == null:
+		drag_target = get_parent() as Node2D
+
+
+func _input(event: InputEvent) -> void:
+	if event is InputEventScreenTouch:
+		_handle_press(event.pressed, event.position)
+	elif event is InputEventScreenDrag and _is_dragging:
+		_move_to(event.position)
+	elif event is InputEventMouseButton and event.button_index == MOUSE_BUTTON_LEFT:
+		_handle_press(event.pressed, event.position)
+	elif event is InputEventMouseMotion and _is_dragging:
+		_move_to(event.position)
+
+
+func _handle_press(pressed: bool, screen_pos: Vector2) -> void:
+	if pressed and _is_position_over_target(screen_pos):
+		_start_drag(screen_pos)
+	elif not pressed and _is_dragging:
+		_end_drag(screen_pos)
+
+
+func _start_drag(screen_pos: Vector2) -> void:
+	_is_dragging = true
+	var world_pos: Vector2 = _screen_to_world(screen_pos)
+	_drag_offset = drag_target.global_position - world_pos
+	_original_z_index = drag_target.z_index
+	_original_scale = drag_target.scale
+	drag_target.z_index = DRAG_Z_INDEX
+	drag_target.scale = _original_scale * DRAG_SCALE
+	drag_picked_up.emit(drag_target.global_position)
+
+
+func _end_drag(_screen_pos: Vector2) -> void:
+	_is_dragging = false
+	drag_target.z_index = _original_z_index
+	drag_target.scale = _original_scale
+	drag_released.emit(drag_target.global_position)
+
+
+func _move_to(screen_pos: Vector2) -> void:
+	var world_pos: Vector2 = _screen_to_world(screen_pos)
+	drag_target.global_position = world_pos + _drag_offset
+
+
+func _screen_to_world(screen_pos: Vector2) -> Vector2:
+	var canvas_transform: Transform2D = drag_target.get_viewport().get_canvas_transform()
+	return canvas_transform.affine_inverse() * screen_pos
+
+
+func _is_position_over_target(screen_pos: Vector2) -> bool:
+	if drag_target == null:
+		return false
+	var world_pos: Vector2 = _screen_to_world(screen_pos)
+	var local_pos: Vector2 = drag_target.to_local(world_pos)
+	var area: Area2D = drag_target.get_node_or_null("CollisionArea") as Area2D
+	if area == null:
+		return local_pos.length() < DEFAULT_DRAG_RADIUS
+	for child in area.get_children():
+		if child is CollisionShape2D and child.shape != null:
+			return child.shape.get_rect().has_point(local_pos)
+	return local_pos.length() < DEFAULT_DRAG_RADIUS

scripts/systems/hud.gd

@@ -0,0 +1,20 @@
+## HUD — heads-up display with back button and music toggle.
+extends CanvasLayer
+
+const MUSIC_ON_SYMBOL: String = "♪"
+const MUSIC_OFF_SYMBOL: String = "✕"
+
+var _music_enabled: bool = true
+
+
+func _on_back_button_pressed() -> void:
+	get_tree().quit()
+
+
+func _on_music_toggle_pressed() -> void:
+	_music_enabled = not _music_enabled
+	var volume: float = AudioManager.DEFAULT_MUSIC_VOLUME if _music_enabled else 0.0
+	AudioManager.set_music_volume(volume)
+	var btn: Button = get_node_or_null("MusicToggle") as Button
+	if btn != null:
+		btn.text = MUSIC_ON_SYMBOL if _music_enabled else MUSIC_OFF_SYMBOL