fix: tune real weather particle rendering

shaders/particle_gpu_validation.fragment.glsl

@@ -35,6 +35,12 @@ bool validFieldUv(vec2 uv) {
     return all(greaterThanEqual(uv, vec2(0.0))) && all(lessThanEqual(uv, vec2(1.0)));
 }
 
+const float EARTH_CIRCUMFERENCE_METERS = 40075016.68557849;
+
+float mercatorLatitudeRadians(float worldY) {
+    return atan(sinh(3.141592653589793 * (1.0 - 2.0 * worldY)));
+}
+
 vec3 weatherSpeedColor(float speed) {
     // Fixed 0..30 m/s scale: no per-frame or per-dataset auto stretching.
     float t = clamp(speed / 30.0, 0.0, 1.0);
@@ -83,10 +89,16 @@ void main() {
             position = seed;
             velocity = vec2(0.0);
         }
-        // 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 = position + mercatorVelocity * u_pass.y * 0.00004;
+        // U/V remain canonical metres per second. u_pass.y is the explicit
+        // visual simulation delta in seconds; the validation clock may run
+        // faster than wall time, but it never changes the field contract.
+        // Web Mercator x spans Earth's circumference; y additionally scales
+        // with sec(latitude). V is north-positive while world y is southward.
+        float latitude = mercatorLatitudeRadians(position.y);
+        float dx = velocity.x * u_pass.y / EARTH_CIRCUMFERENCE_METERS;
+        float dy = -velocity.y * u_pass.y /
+                   (EARTH_CIRCUMFERENCE_METERS * max(cos(latitude), 0.01));
+        position = position + vec2(dx, dy);
         if (!validFieldUv(fieldUv(position)) || any(isnan(velocity))) position = seed;
         fragColor = vec4(position, fract(previous.b + 0.03125 * u_pass.y), 1.0);
     } else if (mode < 1.5) {