test: add deposit-fold bench and final gate

Fix field_reassign_with_default clippy errors in deposit test code
(config.rs and simulation/mod.rs); add fold_deposits_sqrt bench case
to diffusion_benchmark.rs. All 5 gate steps pass: fmt clean, clippy
zero warnings, 800 tests pass, 812 all-features pass, 47 goldens
unchanged.

Author: tamirelazar

benches/diffusion_benchmark.rs

@@ -257,13 +257,44 @@ fn bench_afterglow_ema_pass(c: &mut Criterion) {
     });
 }
 
+fn bench_fold_deposits_sqrt(c: &mut Criterion) {
+    use tslime::simulation::config::DepositCurve;
+    use tslime::simulation::trail_map::TrailMap;
+
+    let width = 400;
+    let height = 400;
+    let mut tm = TrailMap::new(width, height);
+    // Populate the accumulation buffer with non-zero values.
+    let accum = tm.accum_mut();
+    for (i, v) in accum.iter_mut().enumerate() {
+        *v = (i * 7 % 100) as f32 / 10.0;
+    }
+
+    c.bench_function("fold_deposits_sqrt", |b| {
+        b.iter(|| {
+            // Re-fill accum each iteration so fold_deposits has work to do.
+            let accum = tm.accum_mut();
+            for (i, v) in accum.iter_mut().enumerate() {
+                *v = (i * 7 % 100) as f32 / 10.0;
+            }
+            tm.fold_deposits(
+                black_box(DepositCurve::Sqrt),
+                black_box(1.0),
+                black_box(1.0),
+                black_box(0.0),
+            );
+        });
+    });
+}
+
 criterion_group!(
     benches,
     bench_diffuse_mean3x3,
     bench_diffuse_gaussian,
     bench_diffuse_comparison,
     bench_diffuse_gaussian_separable_sigma3,
     bench_decay_gamma,
-    bench_afterglow_ema_pass
+    bench_afterglow_ema_pass,
+    bench_fold_deposits_sqrt
 );
 criterion_main!(benches);

src/simulation/config.rs

@@ -2731,21 +2731,29 @@ mod tests {
         assert_eq!(cfg.deposit_cap, 0.0);
         assert!(!cfg.deposit_active(), "defaults must be the off path");
 
-        let mut on = SimConfig::default();
-        on.deposit_curve = DepositCurve::Sqrt;
+        let on = SimConfig {
+            deposit_curve: DepositCurve::Sqrt,
+            ..Default::default()
+        };
         assert!(on.deposit_active());
-        let mut scaled = SimConfig::default();
-        scaled.deposit_scale = 2.0;
+        let scaled = SimConfig {
+            deposit_scale: 2.0,
+            ..Default::default()
+        };
         assert!(scaled.deposit_active());
-        let mut capped = SimConfig::default();
-        capped.deposit_cap = 5.0;
+        let capped = SimConfig {
+            deposit_cap: 5.0,
+            ..Default::default()
+        };
         assert!(capped.deposit_active());
     }
 
     #[test]
     fn deposit_validation_rejects_out_of_range() {
-        let mut cfg = SimConfig::default();
-        cfg.deposit_gamma = 0.0; // below MIN_DEPOSIT_GAMMA
+        let cfg = SimConfig {
+            deposit_gamma: 0.0, // below MIN_DEPOSIT_GAMMA
+            ..Default::default()
+        };
         assert!(cfg.validate().is_err());
     }
 }

src/simulation/mod.rs

@@ -2030,10 +2030,12 @@ mod tests {
         let off_cfg = SimConfig::default();
         assert!(!off_cfg.deposit_active(), "off path sanity check");
 
-        let mut on_cfg = SimConfig::default();
-        on_cfg.deposit_curve = DepositCurve::Linear;
-        on_cfg.deposit_scale = 1.0;
-        on_cfg.deposit_cap = 1.0e9; // huge cap → never clips
+        let on_cfg = SimConfig {
+            deposit_curve: DepositCurve::Linear,
+            deposit_scale: 1.0,
+            deposit_cap: 1.0e9, // huge cap → never clips
+            ..Default::default()
+        };
         assert!(
             on_cfg.deposit_active(),
             "cap > 0.0 must activate the accum path"
@@ -2067,8 +2069,10 @@ mod tests {
     #[test]
     fn deposit_sqrt_curve_changes_output() {
         let mut base = Simulation::new(40, 40, SimConfig::default(), 42, InitMode::Random, 0);
-        let mut cfg = SimConfig::default();
-        cfg.deposit_curve = crate::simulation::config::DepositCurve::Sqrt;
+        let cfg = SimConfig {
+            deposit_curve: crate::simulation::config::DepositCurve::Sqrt,
+            ..Default::default()
+        };
         let mut curved = Simulation::new(40, 40, cfg, 42, InitMode::Random, 0);
         for _ in 0..30 {
             base.update(1.0);