test: add particle gpu ping-pong validation

shaders/particle_gpu_validation.fragment.glsl

@@ -0,0 +1,30 @@
+layout (std140) uniform ParticleGpuValidationUBO {
+    highp vec4 u_pass;
+    highp vec4 u_marker;
+};
+
+uniform sampler2D u_previous;
+in vec2 v_uv;
+
+void main() {
+    vec4 previous = texture(u_previous, v_uv);
+    float mode = u_pass.x;
+
+    if (mode < 0.5) {
+        // State pass: deterministic GPU-only state evolution.
+        fragColor = vec4(fract(previous.r + 0.03125), v_uv, 1.0);
+    } else if (mode < 1.5) {
+        // Trail pass: fade the previous trail and add a moving validation marker.
+        float marker = 1.0 - smoothstep(0.0, u_marker.z, distance(v_uv, u_marker.xy));
+        vec3 color = previous.rgb * u_marker.w + marker * vec3(0.08, 0.55, 1.0);
+        fragColor = vec4(min(color, vec3(1.0)), 1.0);
+    } else {
+        // Main pass: composite the latest trail without hiding the validation map.
+        float intensity = max(previous.r, max(previous.g, previous.b));
+        fragColor = vec4(previous.rgb * 0.72, min(intensity * 0.72, 0.72));
+    }
+
+#ifdef OVERDRAW_INSPECTOR
+    fragColor = vec4(1.0);
+#endif
+}