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/layer1.rs

@@ -35,8 +35,7 @@ pub struct Layer1Output {
 
 /// Run the Layer-1 topography pipeline for a single body.
 pub fn run_layer1(hm: &BodyHeightmap) -> Layer1Output {
-    let drainage: DrainageResult =
-        drainage::analyze(&hm.data, hm.width, hm.height, hm.sea_level);
+    let drainage: DrainageResult = drainage::analyze(&hm.data, hm.width, hm.height, hm.sea_level);
     let ta: TerrainAnalysis = TerrainAnalysis::analyze(hm, &drainage);
 
     let raw = features::extract_attractors(hm, &drainage, &ta);
@@ -162,18 +161,21 @@ mod tests {
                 b.terrain_modification_cost.to_bits()
             );
         }
-        assert_eq!(
-            o1.river_network.river_cells,
-            o2.river_network.river_cells
-        );
+        assert_eq!(o1.river_network.river_cells, o2.river_network.river_cells);
     }
 
     #[test]
     fn produces_attractors_and_costs() {
         let o = run_layer1(&hm(256, 128));
         assert!(!o.attractors.is_empty(), "expected some attractors");
-        assert!(o.attractors.iter().all(|a| a.terrain_modification_cost >= 1.0));
-        assert!(o.attractors.iter().all(|a| (0.0..=1.0).contains(&a.strength)));
+        assert!(o
+            .attractors
+            .iter()
+            .all(|a| a.terrain_modification_cost >= 1.0));
+        assert!(o
+            .attractors
+            .iter()
+            .all(|a| (0.0..=1.0).contains(&a.strength)));
     }
 
     #[test]