model: progress of long commands, reported by the solvers and the tet mesher

fieldcraft_model::progress: a host installs a Progress on the worker thread
and reads its snapshot from another; the engine names the step being solved,
the nonlinear solver its load factor, the explicit solver its time, the
fluid solver its steps, transient heat its increments and the tet mesher its
stage and pass. Without one installed every report is a no-op (#322, #357).

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

crates/solver/src/heat.rs

@@ -381,7 +381,9 @@ fn run(model: &Model, resolved: &Resolved, step: &Step, limits: &Limits) -> Resu
             let mut previous = t.clone();
             let (mut frames, mut times) = (vec![t.clone()], vec![0.0]);
             let mut rhs = vec![0.0; n];
+            fieldcraft_model::progress::stage("Time increment", Some(steps as f64), "");
             for i in 1..=steps {
+                fieldcraft_model::progress::count((i - 1) as f64);
                 b.mul(&t, &mut rhs);
                 for (r, fj) in rhs.iter_mut().zip(&f) {
                     *r += dt * fj;

crates/solver/src/nonlinear.rs

@@ -1231,7 +1231,9 @@ fn solve_on(model: &Model, step: &str, limits: &Limits, notes: Vec<String>) -> R
         // The last converged increment's free motion and the change of the load factor over it.
         let mut previous: Option<(Vec<f64>, f64)> = None;
         let max_increments = usize::try_from(settings.max_increments).unwrap_or(usize::MAX);
+        fieldcraft_model::progress::stage("Load factor", Some(1.0), "");
         while t < 1.0 {
+            fieldcraft_model::progress::count(lambda);
             if history.increments.len() >= max_increments {
                 return Err(Error::Limit(format!(
                     "step `{}`: {} increments reached the step's cap at load factor {lambda:.6} (raise max_increments)",