- Add algebra/rotor.py: word_transition_rotor() as a free operator function
- Update algebra/__init__.py: export word_transition_rotor
- Refactor vocab/manifold.py: remove edge_rotor(), add versor grade-norm
invariant check in add() to reject raw coordinate vectors at insertion time
VocabManifold now stores only algebraically valid Cl(4,1) versors and
exposes only relational lookup (CGA inner product). Rotor construction
is a contextual algebra-layer concern, not a vocabulary property.
