fix(scene): remove exit-gate flashes and sub-3 Hz-unsafe flickers

src/scene/world/districts.ts

@@ -224,11 +224,11 @@ export function createDistricts(
           varying float vRise;
           void main() {
             ${FADES}
-            // Neon tubes catch as the district finishes rising: a brief
-            // stutter, then steady (no per-instance CPU work).
+            // Neon tubes warm up as the district finishes rising. A smooth
+            // ignite, never a stutter: nothing in the scene flashes faster
+            // than 3 Hz (photosensitivity, round 5).
             float on = smoothstep(0.8, 1.0, vRise);
-            float stutter = step(0.35, fract(sin(floor(uTime * 22.0) + vWorld.z * 0.07) * 43758.5));
-            on *= mix(stutter, 1.0, step(0.985, vRise));
+            on *= on;
             vec3 tone = mix(vec3(0.55, 0.95, 1.0), uAccent, ${accentMix.toFixed(2)});
             // Close passes dim, so overhead neon never floods the bloom pass.
             float near = mix(0.3, 1.0, smoothstep(5.0, 32.0, vDepth));