STEP import: 65/66 Fusion files; bounded meshing

Meshing (vendored truck-meshalgo) works within a budget so hostile files
cannot hang: bounded edge and surface divisions (point caps, depth and
round limits, NaN-safe), and a grid budget per face shared out per shell.
Surface grids refine only the direction that bends; rulings left undivided
get cells comparable to the other direction, so trimmed cylinders (holes,
tee joints) no longer lose volume to long triangles. The helical sweep
imports in 12 s instead of 7 minutes.

Small sphere patches get poles away from them even when the file runs a
seam through the patch (a pyramid's corner blend). Faces the mesher cannot
mesh are reported as import warnings. Test: wild B-spline and far-off
boundaries import in bounded time.

STEP corpus 65/66 (50-sweep-tight-bend: a horn-torus bend whose inner side
collapses to a point; Fusion's own area for it is also off).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/kernel/src/step_in/mod.rs

@@ -233,6 +233,8 @@ struct Reader<'a> {
     node_items: HashSet<u64>,
     /// Occurrences and mapped representations visited so far.
     visits: usize,
+    /// Items whose meshing was checked.
+    unmeshed_checked: HashSet<u64>,
     bodies: Vec<ImportedBody>,
 }
 
@@ -454,6 +456,13 @@ impl<'a> Reader<'a> {
                         Ok(body) => {
                             let own = e.params().first().and_then(Param::as_str).unwrap_or("").trim().to_string();
                             let name = if own.is_empty() { product.to_string() } else { own };
+                            // Faces the mesher gives up on (or that the budget cut short) are reported.
+                            if self.unmeshed_checked.insert(item) {
+                                let n = body.unmeshed_faces();
+                                if n > 0 {
+                                    self.warn(format!("solid '{name}': {n} face(s) could not be meshed and are not shown"));
+                                }
+                            }
                             out.push(self.bodies.len());
                             let color = self.colors.get(&item).copied();
                             self.bodies.push(ImportedBody { name, body: body.with_color(color), color, path: path.to_vec(), closed, file_faces });
@@ -682,6 +691,7 @@ pub fn step_import(text: &str) -> Result<StepImport> {
         used_items: HashSet::new(),
         node_items: HashSet::new(),
         visits: 0,
+        unmeshed_checked: HashSet::new(),
         bodies: Vec::new(),
     };
     rd.index();