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/model/src/check.rs

@@ -322,6 +322,7 @@ impl Model {
                     Procedure::ExplicitDynamic(_) => "is dynamic",
                     Procedure::HeatTransfer { transient: Some(_) } => "is transient heat transfer",
                     Procedure::Static {}
+                    | Procedure::Buckle { .. }
                     | Procedure::NonlinearStatic(_)
                     | Procedure::Fluid { .. }
                     | Procedure::HeatTransfer { transient: None }
@@ -486,7 +487,7 @@ impl Model {
                 }
             }
             match &s.procedure {
-                Procedure::Static {} | Procedure::NonlinearStatic(_) => {
+                Procedure::Static {} | Procedure::NonlinearStatic(_) | Procedure::Buckle { .. } => {
                     if s.loads.is_empty() && !prescribes_motion {
                         d.warning(&subject, "nothing loads the model in this step");
                     }