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/assets/models/nebula_planet.glb.import

@@ -0,0 +1,45 @@
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+
+importer="scene"
+importer_version=1
+type="PackedScene"
+uid="uid://bpfwv7fse77wk"
+path="res://.godot/imported/nebula_planet.glb-2431590907ff85cf1057e7d6ad614ed7.scn"
+
+[deps]
+
+source_file="res://assets/models/nebula_planet.glb"
+dest_files=["res://.godot/imported/nebula_planet.glb-2431590907ff85cf1057e7d6ad614ed7.scn"]
+
+[params]
+
+nodes/root_type=""
+nodes/root_name=""
+nodes/root_script=null
+mesh_library/use_node_names_as_mesh_names=false
+array_mesh/deduplicate_surfaces=true
+nodes/apply_root_scale=true
+nodes/root_scale=1.0
+nodes/import_as_skeleton_bones=false
+nodes/use_name_suffixes=true
+nodes/use_node_type_suffixes=true
+meshes/ensure_tangents=true
+meshes/generate_lods=true
+meshes/create_shadow_meshes=true
+meshes/light_baking=1
+meshes/lightmap_texel_size=0.2
+meshes/force_disable_compression=false
+skins/use_named_skins=true
+animation/import=true
+animation/fps=30
+animation/trimming=false
+animation/remove_immutable_tracks=true
+animation/import_rest_as_RESET=false
+import_script/path=""
+materials/extract=0
+materials/extract_format=0
+materials/extract_path=""
+_subresources={}
+gltf/naming_version=2
+gltf/embedded_image_handling=1
+gltf/texture_map_mode=1