Reconcile the fast projective arithmetic with the CoordinateSystem abstraction

[daira]

#13 deliberately keeps PVes a bare triple with proven commutation rather than an instance of the planned CoordinateSystem abstraction (see the note in #13's description). When the abstraction lands, reconcile the two: the reusable pieces are the RCB completeness argument, the toAffine/ofAffine bridge shape, and the homogeneity lemmas. The fast tier should become (or be derivable from) an instance, so downstream consumers get one interface instead of a parallel API.

🤖 Claude Fable 5