fix: keep particle density stable across zoom

shaders/particle_gpu_validation.fragment.glsl

@@ -4,6 +4,7 @@ layout (std140) uniform ParticleGpuValidationUBO {
     highp vec4 u_mask;
     highp vec4 u_visual;
     highp vec4 u_field_bounds;
+    highp vec4 u_respawn_bounds;
     highp vec4 u_field_scale;
     highp mat4 u_matrix;
 };
@@ -57,7 +58,12 @@ float particleHash(vec2 value) {
 vec2 respawnSeed(vec2 particleUV, vec2 entropy, float attempt) {
     vec2 salt = entropy * vec2(13.17, 19.91) + particleUV * vec2(97.23, 53.71) + attempt;
     vec2 random = vec2(particleHash(salt), particleHash(salt.yx + 41.29));
-    return mix(u_field_bounds.xy, u_field_bounds.zw, random);
+    return mix(u_respawn_bounds.xy, u_respawn_bounds.zw, random);
+}
+
+bool outsideRespawnBounds(vec2 position) {
+    return position.x < u_respawn_bounds.x || position.x > u_respawn_bounds.z ||
+           position.y < u_respawn_bounds.y || position.y > u_respawn_bounds.w;
 }
 
 bool fieldVelocityAt(vec2 position, out vec2 velocity) {
@@ -111,7 +117,11 @@ void main() {
             vec2 position = previous.rg;
             vec2 velocity;
             bool valid = !respawn && fieldVelocityAt(position, velocity);
-            if (!valid || age >= maxAge) {
+            // The field bounds remain the only sampling domain. Once the
+            // camera is stable, slots already outside the expanded viewport
+            // may recycle into its separate preferred respawn domain.
+            if (!valid || age >= maxAge ||
+                (u_field_scale.z > 0.5 && !initialSeed && outsideRespawnBounds(position))) {
                 respawn = true;
             }