Future improvements to hard-edge fringes

- [ ] Curved magnet fringe. Should try to find an explicit map which reproduces Eq. (13.8) of Etienne's yellow book to leading order. Will also need to first drift each particle to the magnet edge. Once curved multipoles are added, should also add effects of other orders.
- [ ] Fringe for sextupole and above. Should use Eq. (13.31) from Etienne's yellow book.
- [ ] Once electric multipoles of all orders are added using implicit, they need spin fringes. Just use $`\int E_s\,\mathrm{d}s=-\Phi`$.

Start by tracing the existing hard-edge fringe, curved multipole, and implicit electric-multipole entry points in the Julia package. Read the cited equations (13.8 and 13.31) in Etienne's yellow book before deciding how the requested maps and spin fringes fit together. Done means all three checklist areas are implemented, including edge drifts, higher-order multipoles, and the stated electric spin-fringe integral.

Written by the indexing model from the issue text.