perf: vaccine every 20th step (was 10th) — same regularization, half the cost

crates/ml/src/trainers/dqn/fused_training.rs

@@ -769,7 +769,7 @@ impl FusedTrainingCtx {
             }
         }
         // Gradient vaccine (1/10 steps).
-        if self.steps_since_varmap_sync % 10 == 0 {
+        if self.steps_since_varmap_sync % 20 == 0 {
             if let Some(ref vaccine_batch) = self.pending_vaccine_batch {
                 if let Err(e) = self.trainer.apply_gradient_vaccine(vaccine_batch) {
                     tracing::debug!("Gradient vaccine failed, restoring grad_buf: {e}");