rgsaddle

Band and minimum-mode saddle mechanics over rgmin steppers.

Docs: https://omnipotentrpc.github.io/rgsaddle/

Two seams, stepping at both. The inner seam is rgmin's Solver: one optimizer step over an assembled band force; this crate defines no optimizer of its own. The outer seam is BandSession::step: assemble NEB forces on the caller's surface, take one solver step, report. No run-to-completion contract exists; hosts own the loop and interleave their policy (trust regions, drift budgets, acquisition, hybrid MMF) between steps. run is a convenience loop over step and nothing more.

Force assembly ports eOn's NEB mechanics with identical branch structure: tangents (Mills-Jonsson-Schenter simple, Henkelman-Jonsson improved), springs (uniform, energy-weighted, Onsager-Machlup), projections (plain elastic band, NEB, doubly nudged), and the climbing-image force with the eOn trigger rule.

Positions are unwrapped Cartesian. A Cell supplies orthorhombic minimum-image differences for the band mechanics; neighbor lists, when a surface wants them, are vesin's job, not this crate's.

BandSession::reset is the model-update boundary: quasi-Newton history taken on one surface epoch must not survive onto the next.