feat(validation): add stratified sampling by ring size

tests/validation/test_sampling.py

@@ -0,0 +1,23 @@
+import pandas as pd
+import pytest
+
+from macro_lactone_toolkit.validation.sampling import stratified_sample_by_ring_size
+
+
+def test_stratified_sample():
+    # Create test data with known ring sizes
+    data = {
+        "smiles": [
+            "O=C1CCCCCCCCCCCCCCO1",  # 16-membered
+            "O=C1CCCCCCCCCCCCO1",     # 14-membered
+            "O=C1CCCCCCCCCCCCCCCCO1", # 18-membered
+        ],
+        "id": ["A", "B", "C"],
+    }
+    df = pd.DataFrame(data)
+
+    sampled = stratified_sample_by_ring_size(df, sample_ratio=0.5, random_state=42)
+
+    # Should get at least 1 from each ring size (50% of 1 = 1)
+    assert len(sampled) >= 1
+    assert len(sampled) <= 3