test: pin down rng semantics for spring layouts

Adds explicit assertions for the four reproducibility cases Max asked
about: reusing a rng advances state (different output), fresh rngs
from the same seed match, repeated int seeds match, but an int seed
is not equivalent to a rng built from that same int (different
conversion path through _to_nx_seed).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: leotrs

tests/drawing/test_layout.py

@@ -270,3 +270,41 @@ def test_spring_layouts_accept_rng(edgelist1):
     xgi.barycenter_spring_layout(H, seed=42)
     # None still works
     xgi.barycenter_spring_layout(H, seed=None)
+
+
+def test_spring_layouts_rng_semantics(edgelist1):
+    """Document expected reproducibility semantics for the seed argument.
+
+    The conventions mirror sklearn / scipy:
+
+    * Reusing the same `Generator` instance advances its state, so two
+      consecutive calls produce different layouts.
+    * Two fresh `Generator`s constructed from the same seed produce identical
+      layouts.
+    * Passing an int seed reproduces itself, but is not equivalent to passing a
+      `Generator` constructed from that same int (the conversion path differs).
+    """
+    H = xgi.Hypergraph(edgelist1)
+
+    # Reusing one rng across calls: state advances → different outputs
+    rng = np.random.default_rng(42)
+    pos_a = xgi.barycenter_spring_layout(H, seed=rng)
+    pos_b = xgi.barycenter_spring_layout(H, seed=rng)
+    assert any(not np.allclose(pos_a[n], pos_b[n]) for n in pos_a)
+
+    # Two fresh rngs from the same seed → identical outputs
+    pos_c = xgi.barycenter_spring_layout(H, seed=np.random.default_rng(42))
+    pos_d = xgi.barycenter_spring_layout(H, seed=np.random.default_rng(42))
+    for n in pos_c:
+        assert np.allclose(pos_c[n], pos_d[n])
+
+    # Same int seed reused → identical outputs
+    pos_e = xgi.barycenter_spring_layout(H, seed=42)
+    pos_f = xgi.barycenter_spring_layout(H, seed=42)
+    for n in pos_e:
+        assert np.allclose(pos_e[n], pos_f[n])
+
+    # int seed vs rng built from same seed → NOT equivalent (different paths)
+    pos_g = xgi.barycenter_spring_layout(H, seed=42)
+    pos_h = xgi.barycenter_spring_layout(H, seed=np.random.default_rng(42))
+    assert any(not np.allclose(pos_g[n], pos_h[n]) for n in pos_g)