chore: silence clippy nits in new SBX/PolyMut code

- PolynomialMutation::vary: `#[allow(clippy::needless_range_loop)]`
  on the per-dimension loop — body indexes both `self.bounds[j]` and
  `child[j]` so a range index is the cleanest option.
- Operator tests: replace `x >= lo && x <= hi` with
  `(lo..=hi).contains(&x)` per clippy's manual_range_contains lint.

src/operators/composite.rs

@@ -75,7 +75,7 @@ mod tests {
         for c in &children {
             assert_eq!(c.len(), 4);
             for &x in c {
-                assert!(x >= -1.0 && x <= 1.0);
+                assert!((-1.0..=1.0).contains(&x));
             }
         }
     }