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

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

shaders/caustics.gdshader

@@ -0,0 +1,21 @@
+shader_type spatial;
+render_mode unshaded, blend_add, cull_disabled, depth_draw_never;
+
+uniform float speed : hint_range(0.1, 3.0) = 0.8;
+uniform float scale : hint_range(1.0, 20.0) = 8.0;
+uniform float intensity : hint_range(0.0, 2.0) = 0.6;
+uniform vec3 caustic_color : source_color = vec3(0.7, 0.9, 1.0);
+
+void fragment() {
+	vec2 uv = UV * scale;
+
+	float pattern1 = sin(TIME * speed + uv.x * 10.0 + sin(uv.y * 10.0 + TIME * speed));
+	float pattern2 = sin(TIME * speed * 0.7 + uv.y * 12.0 + sin(uv.x * 8.0 + TIME * speed * 1.3));
+	float pattern3 = sin(TIME * speed * 1.1 + (uv.x + uv.y) * 7.0 + sin(uv.x * 5.0 - TIME * speed * 0.5));
+
+	float caustic = (pattern1 + pattern2 + pattern3) / 3.0;
+	caustic = pow(max(caustic, 0.0), 2.0) * intensity;
+
+	EMISSION = caustic_color * caustic;
+	ALBEDO = vec3(0.0);
+}