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

docs/features.md

@@ -123,7 +123,7 @@ the control channel and AI agents over MCP (docs/mcp.md) run the same commands.
 | Decks | Nastran Deck — Write Nastran bulk data (SOL 101, 103) | ready | `deck.write {"format":"nastran"}` |
 | Decks | LS-DYNA Deck — Write an LS-DYNA keyword file | ready | `deck.write {"format":"lsdyna"}` |
 | Decks | Read Deck — Read a deck, keeping keywords FieldCraft does not know | coming soon (M9) |  |
-| Implicit | Static — Linear static (or coupled temperature–displacement) solve of a step with FieldCraft's own solver; nonlinear static and heat transfer in the drop-down | ready | `solve.static`, `solve.nonlinear`, `solve.heat` |
+| Implicit | Static — Linear static (or coupled temperature–displacement) solve of a step with FieldCraft's own solver; nonlinear static, buckling and heat transfer in the drop-down | ready | `solve.static`, `solve.nonlinear`, `solve.buckle`, `solve.heat` |
 | Implicit | Frequency — Natural frequencies and mode shapes of a frequency step | ready | `solve.frequency` |
 | Explicit | Explicit — Transient dynamics of an explicit step by central differences (contact and erosion coming in M10) | ready | `solve.explicit` |
 | Explicit | Blast & Impact — EOS-driven fluids with fluid–structure contact (ALE or SPH) | coming soon (M14) |  |