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/shaders/sky_asteroid_field.gdshader

@@ -0,0 +1,17 @@
+shader_type sky;
+
+// Procedural dusty void: dim rocky-belt haze with a warm horizon glow, sparser
+// stars than arena_01 for a grittier, more enclosed feel.
+void sky() {
+	vec3 dir = EYEDIR;
+	vec3 col = vec3(0.015, 0.012, 0.01);
+	col += vec3(0.09, 0.05, 0.02) * pow(1.0 - abs(dir.y), 4.0);
+	vec3 cell = floor(dir * 220.0);
+	float h = fract(sin(dot(cell, vec3(12.9898, 78.233, 45.164))) * 43758.5453);
+	if (h > 0.9985) {
+		vec3 in_cell = fract(dir * 220.0) - 0.5;
+		float star = smoothstep(0.35, 0.0, length(in_cell));
+		col += vec3(star * (0.4 + fract(h * 999.0)) * 0.8);
+	}
+	COLOR = col;
+}

Game/shaders/sky_asteroid_field.gdshader.uid

@@ -0,0 +1 @@
+uid://bg7diy2ywqkbd