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

src/macro_lactone_toolkit/validation/models.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from datetime import datetime
+from datetime import UTC, datetime
 from typing import List, Optional
 
 from sqlalchemy.orm import Mapped, mapped_column, relationship
@@ -40,7 +40,7 @@ class ParentMolecule(SQLModel, table=True):
     num_sidechains: Optional[int] = None
     cleavage_positions: Optional[str] = None
     numbered_image_path: Optional[str] = None
-    created_at: datetime = Field(default_factory=datetime.utcnow)
+    created_at: datetime = Field(default_factory=lambda: datetime.now(UTC))
     processed_at: Optional[datetime] = None
 
 
@@ -72,6 +72,8 @@ class SideChainFragment(SQLModel, table=True):
     fragment_smiles_labeled: str
     fragment_smiles_plain: str
     dummy_isotope: int
+    has_dummy_atom: bool = Field(default=True)
+    dummy_atom_count: int = Field(default=1)
     atom_count: int
     heavy_atom_count: int
     molecular_weight: float
@@ -79,6 +81,26 @@ class SideChainFragment(SQLModel, table=True):
     image_path: Optional[str] = None
 
 
+class FragmentLibraryEntry(SQLModel, table=True):
+    """Unified fragment library entries."""
+
+    __tablename__ = "fragment_library_entries"
+
+    id: Optional[int] = Field(default=None, primary_key=True)
+    source_type: str = Field(index=True)
+    source_fragment_id: Optional[str] = Field(default=None, index=True)
+    source_parent_ml_id: Optional[str] = Field(default=None, index=True)
+    source_parent_chembl_id: Optional[str] = Field(default=None, index=True)
+    cleavage_position: Optional[int] = Field(default=None, index=True)
+    fragment_smiles_labeled: Optional[str] = None
+    fragment_smiles_plain: str
+    has_dummy_atom: bool = Field(default=False)
+    dummy_atom_count: int = Field(default=0)
+    splice_ready: bool = Field(default=False, index=True)
+    original_bond_type: Optional[str] = None
+    created_at: datetime = Field(default_factory=lambda: datetime.now(UTC))
+
+
 class ValidationResult(SQLModel, table=True):
     """Manual validation records."""