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/attractor_matching.rs

@@ -314,7 +314,7 @@ pub fn match_cities(
             placements.push(CityPlacement {
                 city_id: cities[ci].city_id,
                 position: attractors[ai].position,
-                attractor_type: attractors[ai].attractor_type.clone(),
+                attractor_type: attractors[ai].attractor_type,
                 score,
                 synthetic: false,
             });
@@ -369,7 +369,7 @@ pub fn match_cities(
             placements.push(CityPlacement {
                 city_id: cities[ci].city_id,
                 position: attractors[ai].position,
-                attractor_type: attractors[ai].attractor_type.clone(),
+                attractor_type: attractors[ai].attractor_type,
                 score,
                 synthetic: false,
             });