feat(ambience): underwater shaders + audio manager + main menu + CC0 assets

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

shaders/underwater_fog.gdshader

@@ -0,0 +1,21 @@
+shader_type fog;
+
+uniform float surface_y : hint_range(-100.0, 100.0) = 0.0;
+uniform float abyss_depth : hint_range(-200.0, 0.0) = -60.0;
+
+void fog() {
+	float depth_factor = clamp((WORLD_POSITION.y - abyss_depth) / (surface_y - abyss_depth), 0.0, 1.0);
+
+	vec3 surface_color = vec3(0.15, 0.45, 0.6);
+	vec3 abyss_color = vec3(0.02, 0.08, 0.15);
+	vec3 fog_color = mix(abyss_color, surface_color, depth_factor);
+
+	float current_wave = sin(TIME * 0.3 + WORLD_POSITION.x * 0.05) * 0.02
+		+ sin(TIME * 0.17 + WORLD_POSITION.z * 0.04) * 0.01;
+
+	float base_density = mix(0.3, 0.05, depth_factor) + current_wave;
+	base_density = clamp(base_density, 0.0, 0.4);
+
+	ALBEDO = fog_color;
+	DENSITY = base_density;
+}