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/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")