adding buckling analysis

Thank you for your work on this FEM library. It is well designed and very useful for structural analysis tasks.

The library does not currently support structural buckling analysis. This limits its use for stability assessment of slender members and shell structures.

Add linear eigenvalue buckling analysis. Specifically, support for geometric stiffness matrix assembly and solution of the generalized eigenvalue problem to compute critical load factors and corresponding buckling modes.

I don't think this take much work, just adding the buckling formula to the existing axial load reaction after solving.

I am wrong in something? thanks again

No files or tests are named. Start by tracing the existing axial load reaction, geometric stiffness or matrix-assembly entry points, and the current analysis solver. Done means the library assembles geometric stiffness, solves the generalized eigenvalue problem, and exposes critical load factors and buckling modes with validation tests.

Written by the indexing model from the issue text.