Add systematic saddle-point search workflow

[ResPSnTY]

Summary

• add an atomistic-first systematic saddle-point search workflow based on Hessian modes, minimum-mode following, and geodesic NEB
• add a Spirit-aligned GNEB implementation with geodesic tangents/distances, climbing/falling/stationary images, VP and stereographic LBFGS-Atlas solvers, and optional GPU execution through the existing backend mechanism
• add a concise skyrmion SPS example, public API documentation, CairoMakie path plotting, and CPU/CUDA formula-level tests
• preserve the existing NEB implementation and all existing method signatures/default behavior

Output and units

• per-transition energy.txt follows the legacy NEB table convention and stores absolute energies in joules
• distance.txt stores dimensionless geodesic segment distances
• transitions_energy.txt includes absolute joules plus relative joules/meV
• plots show (E - E_min) / meV by default and label the axis Energy (meV), matching the current NEB documentation example
• rejected searches are recorded in attempts.txt without candidate folders; only accepted transitions produce OVF/image folders

Validation

• test/transition/test_transition.jl: CPU/CUDA Hessian 18/18, CPU/CUDA MMF/GNEB 38/38, automatic transition 17/17, plotting/axis units 5/5
• test/transition/test_solvers.jl: CPU/CUDA solver and Spirit formula tests pass
• existing test/test_neb.jl: 22/22 pass; src/neb/ has no diff
• documentation build succeeds locally (apart from the pre-existing missing disk_llg.txt warning)
• full Pkg.test() advances through the legacy suites but is blocked in the existing test/eigen/test_cubic.jl Enzyme dynamic-matrix compilation on Windows/Julia 1.12; the stack does not enter the transition code

References

• Müller et al., PRL 121, 197202 (2018)
• Bessarab et al., CPC 196, 335-347 (2015)
• Henkelman and Jónsson, JCP 113, 9978-9985 (2000)
• Müller et al., Spirit, PRB 99, 224414 (2019)