fix(sonar): use np.nonzero for single-argument np.where (python:S6729)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

core/magazine.py

@@ -2018,7 +2018,7 @@ def _snap_seed_once(coverage, seed: Sequence[int], blocked) -> list[int] | None:
             # not its dilated footprint -- dilation only decides which
             # solid patches count as one figure, it must not itself widen
             # the box.
-            ys, xs = np.where(solid & (labels == best + 1))
+            ys, xs = np.nonzero(solid & (labels == best + 1))
             if xs.size:
                 anchor = [int(xs.min()), int(ys.min()), int(xs.max()), int(ys.max())]
                 # A bounding box is a rectangle, not the component's actual