feat(validation): enforce single-anchor fragments

- skip fused/shared/multi-anchor side systems during extraction
- add fragment library schema and fragment_library.csv export
- make scaffold prep strict for non-spliceable positions

tests/test_splicing_engine.py

@@ -5,7 +5,7 @@ from macro_lactone_toolkit import MacrolactoneFragmenter
 from macro_lactone_toolkit.splicing.engine import splice_molecule
 from macro_lactone_toolkit.splicing.scaffold_prep import prepare_macrolactone_scaffold
 
-from .helpers import build_macrolactone, canonicalize
+from .helpers import build_macrolactone, build_macrolactone_with_fused_side_ring, canonicalize
 
 
 def test_splice_benzene_methyl():
@@ -49,3 +49,14 @@ def test_prepare_scaffold_and_reassemble_fragment():
     product = splice_molecule(scaffold, Chem.MolFromSmiles(fragment.fragment_smiles_labeled), position=5)
 
     assert canonicalize(product) == canonicalize(built.mol)
+
+
+def test_prepare_scaffold_rejects_position_without_single_anchor_fragment():
+    built = build_macrolactone_with_fused_side_ring(side_chains={10: "methyl"})
+
+    with pytest.raises(ValueError, match="Position 5 does not contain a single-anchor fragmentable side chain"):
+        prepare_macrolactone_scaffold(
+            built.smiles,
+            positions=[5],
+            ring_size=16,
+        )