CHALLENGE 2: Gravity, Jumping, and Preventing a Double Jump
====================================================================

TASK
----
Add gravity and jumping to a CharacterBody2D on a static floor,
using is_on_floor() to only allow jumping while grounded. Confirm
the character can't "double jump" by holding the jump key in
mid-air.

SETUP
-----
- A CharacterBody2D "Player" with a CollisionShape2D (real Shape
  assigned).
- A StaticBody2D "Floor" beneath it, with its own CollisionShape2D.
- An Input Map action "jump" bound to the spacebar.

SOLUTION CODE (attached to Player)
--------------------------------------
extends CharacterBody2D

const GRAVITY = 900.0
const JUMP_VELOCITY = -400.0

func _physics_process(delta: float) -> void:
    if not is_on_floor():
        velocity.y += GRAVITY * delta

    if Input.is_action_just_pressed("jump") and is_on_floor():
        velocity.y = JUMP_VELOCITY

    move_and_slide()

WHY THIS WORKS AS AN ANSWER
----------------------------
This is the chapter's own gravity/jump example, and it satisfies the
"no double jump" requirement for two separate reasons working
together:

1. Input.is_action_just_pressed("jump") - not is_action_pressed() -
   only returns true for the single frame the spacebar is first
   pressed down (Chapter 5's own distinction). Holding the key down
   doesn't keep re-triggering it.

2. Even if is_action_just_pressed() somehow fired again mid-air, the
   "and is_on_floor()" condition would block it - is_on_floor() only
   returns true while the CharacterBody2D is actually resting on the
   Floor's own collision shape. The instant the jump happens,
   velocity.y becomes negative (moving up), the character leaves the
   floor, is_on_floor() becomes false, and the jump condition can no
   longer be satisfied at all until gravity brings the character back
   down and move_and_slide() registers it touching the floor again.

Together, these two checks mean a jump can only ever be triggered by
a fresh press while grounded - exactly the "no double jump" behavior
the task asks for, with no extra flag variable needed the way
Chapter 4's "print once" challenge required.