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/scenes/spectate.tscn

@@ -1,7 +1,6 @@
-[gd_scene load_steps=4 format=3]
+[gd_scene load_steps=3 format=3]
 
 [ext_resource type="Script" path="res://scripts/spectate_mode.gd" id="1_s"]
-[ext_resource type="PackedScene" path="res://scenes/arena_01.tscn" id="2_s"]
 [ext_resource type="PackedScene" uid="uid://c8kak2l3m4n5" path="res://scenes/HUD.tscn" id="3_s"]
 
 [node name="Spectate" type="Node3D"]
@@ -9,6 +8,5 @@ script = ExtResource("1_s")
 bot_a_model_path = "res://bots/promoted/easy.json"
 bot_b_model_path = "res://bots/promoted/easy.json"
 
-[node name="Arena" parent="." instance=ExtResource("2_s")]
 
 [node name="HUD" parent="." instance=ExtResource("3_s")]