CHALLENGE 1: A Coin That Waits for Its Sound to Finish
====================================================================

TASK
----
Rebuild the Coin scene from Chapter 6 with an added
AudioStreamPlayer2D. On collection, hide the sprite and disable the
collision shape immediately, play the sound, await its finished
signal, and only then call queue_free().

SCENE STRUCTURE
----------------
Coin (Area2D)
|-- Sprite2D
|-- CollisionShape2D
|-- AudioStreamPlayer2D  <- Stream: any short pickup sound effect

SOLUTION CODE (attached to Coin)
--------------------------------------
extends Area2D

func _ready() -> void:
    body_entered.connect(_on_body_entered)

func _on_body_entered(body: Node2D) -> void:
    if body.is_in_group("player"):
        $CollisionShape2D.set_deferred("disabled", true)
        $Sprite2D.hide()
        $AudioStreamPlayer2D.play()
        await $AudioStreamPlayer2D.finished
        queue_free()

WHY THIS WORKS AS AN ANSWER
----------------------------
The moment the player overlaps the coin, three things happen
immediately, in this order: the CollisionShape2D is disabled (using
set_deferred, since physics shapes can't safely be changed mid-
collision-callback - Godot defers the actual change to the next safe
point), the Sprite2D is hidden, and the sound starts playing. From
the player's perspective, the coin has already visually and
functionally disappeared at this point - it can no longer be
collected again, and it's no longer visible.

The node itself, however, is deliberately kept alive a little
longer: await $AudioStreamPlayer2D.finished pauses this specific
function (not the whole game - Chapter 7's own distinction) until
the AudioStreamPlayer2D's own finished signal fires, which happens
exactly when the sound clip naturally ends. Only then does
queue_free() actually remove the Coin node. If queue_free() were
called immediately instead (as in Chapter 6's simpler version), the
AudioStreamPlayer2D child would be destroyed along with its parent
before the sound had a chance to finish, cutting it off abruptly -
exactly the problem this chapter's own warning describes.