fix: stabilize particle visibility across zoom levels

shaders/particle_gpu_validation.fragment.glsl

@@ -105,7 +105,9 @@ void main() {
             float maxAge = u_marker.z * mix(0.75, 1.25, lifecycleHash);
             float initialAgeHash = particleHash(v_uv + vec2(71.19, 29.47));
             bool respawn = initialSeed;
-            float age = initialSeed ? initialAgeHash * maxAge : previous.b + u_pass.y;
+            // Lifecycle is wall-clock based: zoom-aware visual advection and
+            // forecast playback must not change respawn cadence.
+            float age = initialSeed ? initialAgeHash * maxAge : previous.b + u_pass.w;
             vec2 position = previous.rg;
             vec2 velocity;
             bool valid = !respawn && fieldVelocityAt(position, velocity);