fix: finalize GPU particle time interpolation

shaders/particle_gpu_validation.fragment.glsl

@@ -33,7 +33,10 @@ void main() {
             position = v_uv;
             velocity = vec2(0.0);
         }
-        position = fract(position + velocity * u_pass.y * 0.004);
+        // Field V remains north-positive. Mercator Y increases southward, so
+        // convert only at the physical-velocity -> map-displacement boundary.
+        vec2 mercatorVelocity = vec2(velocity.x, -velocity.y);
+        position = fract(position + mercatorVelocity * u_pass.y * 0.004);
         if (any(isnan(velocity))) position = v_uv;
         fragColor = vec4(position, fract(previous.b + 0.03125 * u_pass.y), 1.0);
     } else if (mode < 1.5) {