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

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

shaders/godrays.gdshader

@@ -0,0 +1,20 @@
+shader_type canvas_item;
+render_mode blend_add;
+
+uniform float ray_count : hint_range(5.0, 40.0) = 20.0;
+uniform float speed : hint_range(0.1, 2.0) = 0.4;
+uniform float intensity : hint_range(0.0, 2.0) = 0.5;
+uniform vec3 ray_color : source_color = vec3(0.5, 0.8, 1.0);
+
+void fragment() {
+	vec2 uv = UV;
+
+	float vertical_fade = pow(1.0 - uv.y, 2.0);
+
+	float ray_pattern = fract(uv.x * ray_count + sin(TIME * speed + uv.y * 5.0) * 0.3);
+	float ray = smoothstep(0.0, 0.15, ray_pattern) * smoothstep(0.4, 0.15, ray_pattern);
+
+	float brightness = ray * vertical_fade * intensity;
+
+	COLOR = vec4(ray_color * brightness, brightness * 0.6);
+}