feat(validation): add isotope tagging utilities

tests/validation/test_isotope_utils.py

@@ -0,0 +1,34 @@
+import pytest
+from rdkit import Chem
+
+from macro_lactone_toolkit.validation.isotope_utils import (
+    build_fragment_with_isotope,
+    extract_isotope_position,
+)
+
+
+def test_build_fragment_with_isotope():
+    # Create a simple test molecule: ethyl group attached to position 5
+    mol = Chem.MolFromSmiles("CCCC(CC)CCC")  # Position 4 (0-indexed) has ethyl
+    assert mol is not None
+
+    side_chain_atoms = [4, 5]  # The ethyl group atoms
+    side_chain_start = 4
+    ring_atom = 3
+    cleavage_pos = 5
+
+    labeled, plain, bond_type = build_fragment_with_isotope(
+        mol, side_chain_atoms, side_chain_start, ring_atom, cleavage_pos
+    )
+
+    assert labeled is not None
+    assert plain is not None
+    assert bond_type == "SINGLE"
+
+    # Check isotope was set
+    extracted_pos = extract_isotope_position(labeled)
+    assert extracted_pos == cleavage_pos
+
+    # Plain should have no isotope
+    extracted_plain = extract_isotope_position(plain)
+    assert extracted_plain == 0