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/mesh/src/octree/mod.rs

@@ -251,6 +251,7 @@ pub fn octree_hex_mesh(surface: &BoundarySurface, field: &dyn SizeField, options
     let mut hints: Vec<P> = Vec::new();
     let mut retries: Vec<String> = Vec::new();
     for attempt in 0..options.attempts {
+        fieldcraft_model::progress::check()?;
         let cap = if attempt == 0 {
             options.max_elements
         } else {