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/engine/src/cmd/mesh.rs

@@ -252,7 +252,9 @@ pub(super) fn mesh_body(
             None => s.design.as_deref().and_then(|d| d.body(&input.name)).map(analytic_surfaces).unwrap_or_default(),
             Some(_) => Vec::new(),
         };
+        fieldcraft_model::progress::stage(&format!("{}: boundary", input.name), None, "");
         let surface = BoundarySurface::new(&input.mesh.positions, &input.mesh.triangles, &input.mesh.tri_face, &surface_options(p, &input.mesh)?)?;
+        fieldcraft_model::progress::stage(&format!("{}: Delaunay", input.name), None, "");
         let tets = tetrahedralize(&surface, cdt)?;
         let mesh = if quality {
             let options = QualityOptions { seeds: input.seeds.clone(), ..options.clone() };