CHALLENGE 3: A Collectible Coin Using Area2D
====================================================================

TASK
----
Build an Area2D "coin" with a CollisionShape2D. Add a CharacterBody2D
player tagged into a "player" group. Connect the coin's
body_entered signal so that walking into it prints "Coin collected!"
and removes the coin from the scene.

SETUP
-----
1. Create a "Coin" node as an Area2D, with a CollisionShape2D child
   (Shape: CircleShape2D, a small radius).
2. Create (or reuse) a "Player" CharacterBody2D with its own
   CollisionShape2D, and a movement script (e.g. Challenge 1's).
3. Select Player in the Scene panel, open the Node panel's "Groups"
   tab, and add it to a group named "player".

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"):
        print("Coin collected!")
        queue_free()

WHY THIS WORKS AS AN ANSWER
----------------------------
This is a direct application of the chapter's own Area2D example.
body_entered is a signal Area2D already provides - connecting it in
_ready() means _on_body_entered runs automatically any time some
other physics body's collision shape overlaps the Coin's own
CollisionShape2D, with that other body passed in as the body
argument.

Checking body.is_in_group("player") before reacting matters because
body_entered would fire for ANY physics body that overlaps the coin
- not just the player. Tagging Player into the "player" group in the
editor (rather than checking, say, body.name == "Player", which
would break the moment the node is renamed) is what lets the coin
correctly ignore anything else that might wander through it - an
enemy, a pushed crate - while still reacting to the actual player.

queue_free() removes the Coin node at the end of the current frame
(Chapter 3), which is why the coin visibly disappears once collected
rather than lingering as an empty, non-functional node in the scene.