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/arena_registry.gd

@@ -0,0 +1,14 @@
+class_name ArenaRegistry
+
+# Single source of truth for available arenas: the Free Play menu dropdown
+# lists these, and Match/Spectate pick one at random each session. Training
+# is exempt — training.tscn keeps its own fixed arena_01 child.
+const ARENAS := [
+	{"name": "Starfield", "path": "res://scenes/arena_01.tscn"},
+	{"name": "Nebula", "path": "res://scenes/arena_02.tscn"},
+	{"name": "Asteroid Field", "path": "res://scenes/arena_03.tscn"},
+]
+
+
+static func random_path() -> String:
+	return ARENAS[randi() % ARENAS.size()]["path"]

Game/scripts/arena_registry.gd.uid

@@ -0,0 +1 @@
+uid://orkh5g5828cv

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())

Game/scripts/game_mode.gd

@@ -26,6 +26,11 @@ func _ready():
 		if child is Arena:
 			arena = child
 			break
+	if not arena:
+		var path := _get_arena_scene_path()
+		if not path.is_empty():
+			arena = (load(path) as PackedScene).instantiate()
+			add_child(arena)
 	if not arena:
 		push_error("GameMode: scene has no Arena child")
 		return
@@ -34,6 +39,13 @@ func _ready():
 	_start()
 
 
+# Virtual: subclasses whose scene has no fixed Arena child override this to
+# pick which arena scene to instantiate in code (see ArenaRegistry). Modes
+# that keep a fixed Arena child (training.tscn) never call this.
+func _get_arena_scene_path() -> String:
+	return ""
+
+
 # Virtual: subclasses spawn their ball/ships/camera here.
 func _start() -> void:
 	pass

Game/scripts/game_settings.gd

@@ -21,3 +21,7 @@ var dev_bot_override_path: String = ""
 # Spectate (bot vs bot) policies chosen in the main menu; empty = mode export.
 var spectate_bot_a_path: String = ""
 var spectate_bot_b_path: String = ""
+
+# Arena chosen in the main menu for Free Play; empty = ArenaRegistry's default.
+# Match/Spectate ignore this and pick a random arena each session instead.
+var selected_arena_path: String = ""

Game/scripts/main_menu.gd

@@ -18,6 +18,7 @@ const DIFFICULTIES := [
 ]
 
 @onready var difficulty_dropdown: OptionButton = %DifficultyDropdown
+@onready var arena_dropdown: OptionButton = %ArenaDropdown
 @onready var dev_section: Control = %DevSection
 @onready var dev_bot_dropdown: OptionButton = %DevBotDropdown
 @onready var bot_a_dropdown: OptionButton = %BotADropdown
@@ -26,6 +27,7 @@ const DIFFICULTIES := [
 
 func _ready() -> void:
 	_populate_difficulty_dropdown()
+	_populate_arena_dropdown()
 	dev_section.visible = OS.is_debug_build()
 	if dev_section.visible:
 		var bots := _list_bots()
@@ -47,6 +49,19 @@ func _populate_difficulty_dropdown() -> void:
 	difficulty_dropdown.select(selected)
 
 
+# Free Play only — Match/Spectate pick a random arena in code instead.
+func _populate_arena_dropdown() -> void:
+	arena_dropdown.clear()
+	for tier in ArenaRegistry.ARENAS:
+		arena_dropdown.add_item(tier["name"])
+	var selected := 0
+	for i in ArenaRegistry.ARENAS.size():
+		if ArenaRegistry.ARENAS[i]["path"] == GameSettings.selected_arena_path:
+			selected = i
+			break
+	arena_dropdown.select(selected)
+
+
 # Lists bot checkpoints as paths relative to BOTS_DIR, including the
 # top-level curriculum checkpoints and anything under promoted/.
 func _list_bots() -> Array[String]:
@@ -93,6 +108,8 @@ func _selected_path(dropdown: OptionButton) -> String:
 
 
 func _on_free_play_pressed() -> void:
+	var chosen: Dictionary = ArenaRegistry.ARENAS[0] if arena_dropdown.selected < 0 else ArenaRegistry.ARENAS[arena_dropdown.selected]
+	GameSettings.selected_arena_path = chosen["path"]
 	get_tree().change_scene_to_file("res://scenes/free_play.tscn")
 
 

Game/scripts/match_mode.gd

@@ -32,6 +32,10 @@ var match_timer: Timer
 var _in_overtime := false
 
 
+func _get_arena_scene_path() -> String:
+	return ArenaRegistry.random_path()
+
+
 func _start() -> void:
 	spawn_ball()
 	var player_ship := spawn_ship(0, 0, PlayerShipController.new())

Game/scripts/spectate_mode.gd

@@ -19,6 +19,10 @@ signal score_changed(score: Dictionary)
 var score := {0: 0, 1: 0}
 
 
+func _get_arena_scene_path() -> String:
+	return ArenaRegistry.random_path()
+
+
 func _start() -> void:
 	spawn_ball()
 	# The main menu's dropdown selections (GameSettings autoload) win over the