MAINT: Port MMF-NEB-CI (RO-NEB-CI) to main

[HaoZeke]

Summary

Port MMF-NEB-CI (RO-NEB-CI) onto main (product trunk, currently v2.15.0 / TheochemUI/eOn).

Context

Originally titled for develop. That branch is a stale v3-era snapshot (PR #238 mergeMainv3, 2025-08) and is not the integration trunk. main is the default branch; all new work targets main.

Acceptance

• Feature available on main with tests / approval coverage comparable to existing NEB optimizers.
• Do not base the PR on develop or v3_*.

Originally noted for develop; retargeted 2026-06-27 in Tier B hygiene.

[HaoZeke]

Tier B (2026-06-27): retargeted from develop to main. develop/v3_* are historical; implement on current trunk only.