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 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/decks/src/lsdyna/mod.rs

@@ -899,7 +899,7 @@ fn write_step(d: &mut Deck, model: &Model, r: &Resolved, plan: &Plan, step: &Ste
                 d.card([Field::Blank, Field::Blank, Field::Blank, Field::Blank, Field::from(1)]);
             }
         }
-        Procedure::ExplicitDynamic(_) | Procedure::HeatTransfer { .. } | Procedure::CoupledTemperatureDisplacement {} => {
+        Procedure::ExplicitDynamic(_) | Procedure::HeatTransfer { .. } | Procedure::CoupledTemperatureDisplacement {} | Procedure::Buckle { .. } => {
             return Err(Error::Unsupported { format: FORMAT, what: format!("step `{}`: {} steps", step.name, step.procedure.name()) });
         }
     }