style(simulation): rustfmt + clippy fixes for Layer-1/heightmap code (#953 #963)

Pre-push surfaced fmt + clippy (-D warnings) failures in the new code:
- rustfmt the 6 new/changed atlas files + the bench example.
- features.rs: HashMap → BTreeMap (project bans HashMap for determinism via
  clippy disallowed_types; the bucket map is lookup-only either way).
- attractor_matching.rs: drop now-redundant .clone() on AttractorType (it
  became Copy in #953) — clippy clone_on_copy.
- drainage.rs tests: manual range → (1..=12).contains(&n).
- features.rs test: drop .clone() on Copy AttractorType.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/src/atlas/drainage.rs

@@ -457,7 +457,10 @@ fn merge_small_basins(
 
     while active.len() > min_count {
         // Smallest active basin (tie → lowest id; BTreeSet iterates ascending).
-        let smallest = *active.iter().min_by_key(|&&b| (size[b as usize], b)).unwrap();
+        let smallest = *active
+            .iter()
+            .min_by_key(|&&b| (size[b as usize], b))
+            .unwrap();
         let smallest_size = size[smallest as usize];
         if active.len() <= max_count && smallest_size as f64 / n as f64 >= min_frac {
             break;
@@ -661,7 +664,7 @@ mod tests {
         let result = analyze(&elev, 128, 64, 0.3);
         let n = result.drainage_basins.len();
         assert!(
-            n >= 1 && n <= 12,
+            (1..=12).contains(&n),
             "Basin count {} out of expected range [1, 12]",
             n
         );
@@ -693,7 +696,10 @@ mod tests {
         let r2 = analyze(&elev, 64, 32, 0.3);
         assert_eq!(r1.flow_accumulation, r2.flow_accumulation);
         assert_eq!(r1.max_accumulation, r2.max_accumulation);
-        assert!(r1.max_accumulation >= 1, "max_accumulation must be clamped ≥ 1");
+        assert!(
+            r1.max_accumulation >= 1,
+            "max_accumulation must be clamped ≥ 1"
+        );
         // Flat / all-ocean world: still well-defined (no division by zero).
         let flat = flat_grid(16, 8, 0.5);
         let rf = analyze(&flat, 16, 8, 0.9); // sea_level above all terrain
@@ -714,6 +720,6 @@ mod tests {
         }
         let res = analyze(&elev, w as u32, h as u32, 0.3);
         let n = res.drainage_basins.len();
-        assert!(n >= 1 && n <= 12, "basin count {n} out of range");
+        assert!((1..=12).contains(&n), "basin count {n} out of range");
     }
 }