Implements extract_dag(), classify_type(), and compute_properties_hash()
for extracting feature trees from FreeCAD documents.
- classify_type: maps ~50 FreeCAD TypeIds to 8 DAG node types
- compute_properties_hash: SHA-256 of per-feature parametric inputs
- extract_dag: two-pass walk of doc.Objects producing nodes + edges
No GUI dependencies -- works in both desktop and headless mode.
Closeskindred/create#214