solver, model, engine: linear buckling steps

Procedure::Buckle {modes, shift}: the step's loads as a linear preload, K_σ from its stresses,
and (K + λ K_σ) φ = 0 by block Lanczos in the K inner product: K⁻¹(−K_σ) on Cholesky without a
shift, the buckling transformation (K + σ K_σ)⁻¹ K on LDLᵀ with one, whose inertia counts the
load factors below σ (Grimes–Lewis–Simon 1994). Lanczos gains an operand and an eigenvalue map
(defaults keep frequency steps bit for bit) and reports converged negative eigenvalues. Results
are frequency results with a `buckling` part; modes display with their load factor.
solve.buckle (in the Static drop-down); Abaqus *BUCKLE, Nastran SOL 105 and LS-DYNA refuse it by
name. Euler columns of hex20 within 0.18 % of Engesser's load.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

docs/solvers/literature.md

@@ -1246,6 +1246,11 @@ update are written from (cited again in the module docs):
   34(1):82–109, 1992.
 - Iterative refinement: N. J. Higham, *Accuracy and Stability of Numerical Algorithms*, 2nd ed.,
   SIAM 2002, §12.2.
+- Linear buckling and the initial-stress stiffness: K.-J. Bathe, *Finite Element Procedures*,
+  2nd ed., 2014, §6.8.2; the buckling spectral transformations in the K inner product: R. G.
+  Grimes, J. G. Lewis, H. D. Simon, SIAM J. Matrix Anal. Appl. 15(1):228–272, 1994, §3; Euler's
+  columns with Engesser's shear correction: S. P. Timoshenko, J. M. Gere, *Theory of Elastic
+  Stability*, 2nd ed., McGraw-Hill 1961, §2.1, §2.17.
 
 ## Part Q: Contact and friction (explicit), as implemented