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/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