engine: parameter schemas for solve.buckle, buckling steps and imperfections

Fixes the build of develop: #171 crossed with the typed command forms, so BUCKLE lacked its
form. SOLVE_BUCKLE follows SOLVE_FREQUENCY; the step form gains the buckle variant and the
nonlinear step's imperfection group.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/cmd/solve.rs

@@ -64,6 +64,7 @@ pub const BUCKLE: CommandSpec = CommandSpec {
     id: "solve.buckle",
     label: "Solve Buckling Step",
     params: r#"{"step": "Buckle", "max_dofs": 6000000, "max_factor_entries": 400000000, "max_iterations": 3000, "threads": 8} — linear buckling of a buckling step (the only one when `step` is left out): the step's loads as a linear preload, then the lowest load factors λ with (K + λ K_σ) φ = 0 and their mode shapes (largest component 1), or those above the step's shift (whose LDLᵀ factor counts the load factors below it); negative load factors found are reported; the result stays in the session and its modes display like frequency modes; `threads` as for solve.static"#,
+    form: Some(super::forms::SOLVE_BUCKLE),
     kind: CommandKind::Model,
     run: run_buckle,
 };