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 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/cmd/forms.rs

@@ -439,8 +439,18 @@ const PROCEDURE: Field = variants(
                 ),
                 flag("line_search", "Line search").def("false"),
                 json("load_path", "Load path").hint("[[fraction, factor], …]; a ramp when empty"),
+                group(
+                    "imperfection",
+                    "Imperfection",
+                    &[
+                        reference("step", "Buckling step", C::Step).req(),
+                        int("mode", "Mode", 1, 100_000).req().def("1"),
+                        num("scale", "Largest component", D::Length).req(),
+                    ],
+                ),
             ],
         ),
+        variant("buckle", "Buckling", &[int("modes", "Modes", 1, 100_000).req().def("4"), num("shift", "Shift", D::None)]),
         variant(
             "frequency",
             "Frequency",
@@ -803,6 +813,7 @@ pub const SOLVE_FREQUENCY: &[Field] = &[
     int("max_iterations", "Max Lanczos vectors", 1, 3000).hint("3000"),
     THREADS,
 ];
+pub const SOLVE_BUCKLE: &[Field] = &[STEP, MAX_DOFS, MAX_FACTOR, int("max_iterations", "Max Lanczos vectors", 1, 3000).hint("3000"), THREADS];
 pub const SOLVE_EXPLICIT: &[Field] = &[
     STEP,
     MAX_DOFS,

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,
 };