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/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