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

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

shaders/caustics.gdshader

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+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);
+}

shaders/godrays.gdshader

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+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);
+}

shaders/underwater_fog.gdshader

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+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;
+}