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

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

crates/explicit/src/integrate.rs

@@ -363,6 +363,7 @@ pub fn solve_explicit(model: &Model, step: Option<&str>, limits: &Limits) -> Res
     loop {
         if increments.is_multiple_of(PROGRESS_EVERY) {
             fieldcraft_model::progress::count(t);
+            fieldcraft_model::progress::check()?;
         }
         if increments >= max_increments {
             completed = false;