feat(world): tiled heightfield terrain actor with per-cell sonar materials

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

game/world/sites/seabed_terrain.gdshader

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+shader_type spatial;
+render_mode cull_disabled, depth_draw_opaque;
+
+uniform vec3 sand_color : source_color = vec3(0.50, 0.46, 0.34);
+uniform vec3 rock_color : source_color = vec3(0.21, 0.20, 0.18);
+uniform vec3 deep_tint : source_color = vec3(0.04, 0.09, 0.15);
+uniform float sand_slope_cosine = 0.9397; // cos(20 deg)
+uniform float rock_slope_cosine = 0.7660; // cos(40 deg)
+uniform float depth_tint_start_m = 20.0;
+uniform float depth_tint_end_m = 90.0;
+uniform float ripple_wavelength_m = 1.6;
+uniform float ripple_contrast = 0.12;
+
+varying vec3 world_position;
+varying vec3 world_normal;
+
+void vertex() {
+	world_position = (MODEL_MATRIX * vec4(VERTEX, 1.0)).xyz;
+	world_normal = normalize((MODEL_MATRIX * vec4(NORMAL, 0.0)).xyz);
+}
+
+void fragment() {
+	float up = clamp(world_normal.y, 0.0, 1.0);
+	float rockness = 1.0 - smoothstep(rock_slope_cosine, sand_slope_cosine, up);
+	vec3 albedo = mix(sand_color, rock_color, rockness);
+	float phase = (world_position.x * 0.8 + world_position.z * 0.6) * TAU / ripple_wavelength_m;
+	float ripple = sin(phase) * ripple_contrast * (1.0 - rockness);
+	albedo *= 1.0 + ripple;
+	float depth_m = -world_position.y;
+	float tint = smoothstep(depth_tint_start_m, depth_tint_end_m, depth_m);
+	albedo = mix(albedo, deep_tint, tint * 0.6);
+	ALBEDO = albedo;
+	ROUGHNESS = mix(0.95, 0.82, rockness);
+	METALLIC = 0.0;
+}