solver: a job cancelled before it starts stops before any setup work

The cancel check came only at the factorization, so on a busy CI runner a
pre-cancelled frequency solve still spent 17 s on setup (cancel_tests).

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

crates/solver/src/buckle.rs

@@ -107,6 +107,8 @@ impl Operator for Shifted<'_> {
 
 /// Solve a buckling step (the step named `step`, or the model's only buckling step).
 pub fn solve_buckle(model: &Model, step: Option<&str>, limits: &Limits) -> Result<FrequencyResult> {
+    // A job cancelled before it starts stops here, before any setup work.
+    fieldcraft_model::progress::check()?;
     let step = model.select_step(step, |p| matches!(p, Procedure::Buckle { .. }), "buckling")?;
     let Procedure::Buckle { modes, shift } = step.procedure else {
         return Err(Error::Internal("buckling step".into()));