solvers, meshers: stop with Cancelled at each loop's natural points

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/mesh/src/tet/improve.rs

@@ -460,6 +460,7 @@ pub fn improve(cx: &mut TetComplex, ctx: &Context, o: &ImproveOptions) -> Result
         stats.passes += 1;
         fieldcraft_model::progress::stage("Improvement pass", Some(o.max_passes as f64), "");
         fieldcraft_model::progress::count(stats.passes as f64);
+        fieldcraft_model::progress::check()?;
         let before = match carried.take() {
             Some(b) => b,
             None => indicators(cx)?,

crates/mesh/src/tet/mod.rs

@@ -491,6 +491,7 @@ pub fn quality_mesh(surface: &BoundarySurface, cdt: &TetMesh, analytic: Vec<Opti
     progress::stage("Refinement round", Some(o.rounds as f64), "");
     for round in 0..o.rounds {
         progress::count((round + 1) as f64);
+        progress::check()?;
         let stats = refine::refine(&mut cx, &ctx, &ropt)?;
         let quiet = stats.interior + stats.boundary == 0;
         let stopped = stats.stopped.clone();
@@ -514,6 +515,7 @@ pub fn quality_mesh(surface: &BoundarySurface, cdt: &TetMesh, analytic: Vec<Opti
     }
     // Put the boundary vertices on the exact surfaces (the tessellation is within its chordal
     // tolerance of them).
+    progress::check()?;
     progress::stage("Snapping to the geometry", None, "");
     report.unsnapped = snap_to_geometry(&mut cx, &exact, true)?;
     let c = cx.compact()?;

crates/mesh/src/tet/refine.rs

@@ -263,6 +263,10 @@ pub fn refine(cx: &mut TetComplex, ctx: &Context, o: &RefineOptions) -> Result<R
     while head < queue.len() {
         let t = queue.get(head).copied().unwrap_or(NONE);
         head += 1;
+        // A round can insert hundreds of thousands of vertices: stop between them when asked.
+        if head.is_multiple_of(1024) {
+            fieldcraft_model::progress::check()?;
+        }
         // Keep the queue's memory bounded: drop the processed prefix now and then.
         if head > 1 << 20 {
             queue.drain(..head);