refactor: extract ModelFeatureFactory from GeologicalModel

Move the seven _build_* feature-construction methods (foliation,
fold frame, folded foliation, folded fold frame, intrusion, domain
fault, fault) -- collectively ~1000 lines, the largest remaining
chunk of GeologicalModel -- into a standalone ModelFeatureFactory.
The FeatureBuilderRegistry wiring at the bottom of geological_model.py
now dispatches to ModelFeatureFactory.build_* instead of the removed
private methods. Public create_and_add_* wrappers are unchanged;
their docstring cross-references are updated to point at the new
factory location. geological_model.py drops from 2579 to 1505 lines.

Author: lachlangrose