feat: add Nebula and Asteroid Field arenas

Introduce arena_02 (Nebula) and arena_03 (Asteroid Field) alongside
arena_01, listed in a new ArenaRegistry (scripts/arena_registry.gd) as
the single source of truth for available arenas. Free Play lets the
player pick an arena from the main menu; Match/Spectate each pick one
at random per session; Training keeps its own fixed arena_01.

Nebula gets an RL-style visual treatment: a near-invisible glass
boundary material shared by all arenas, a baked equirect nebula sky
texture, a drifting-dust particle system, and a Blender-modeled
station/debris/planet decoration set. GameMode gains a
_get_arena_scene_path() hook so modes can instantiate their arena in
code instead of hardcoding it in the scene.

Game/scripts/free_play.gd

@@ -4,6 +4,12 @@ extends GameMode
 # Rocket League's free play. R resets the ball, Esc returns to the menu.
 
 
+func _get_arena_scene_path() -> String:
+	if not GameSettings.selected_arena_path.is_empty():
+		return GameSettings.selected_arena_path
+	return ArenaRegistry.ARENAS[0]["path"]
+
+
 func _start() -> void:
 	spawn_ball()
 	var player_ship := spawn_ship(0, 0, PlayerShipController.new())