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/body.rs

@@ -291,6 +291,21 @@ impl Body {
         })
     }
 
+    /// Faces with no triangles in a coarse tessellation (the mesher failed or gave up).
+    pub fn unmeshed_faces(&self) -> usize {
+        if self.is_mesh() {
+            return 0;
+        }
+        let Ok(m) = self.tessellate((self.size() * 1e-2).max(1e-3)) else { return self.face_count() };
+        let mut has = vec![false; self.face_count()];
+        for f in &m.tri_face {
+            if let Some(h) = has.get_mut(*f as usize) {
+                *h = true;
+            }
+        }
+        has.iter().filter(|h| !**h).count()
+    }
+
     /// Display mesh: like [`Body::tessellate`], with seam edges flagged.
     pub fn display_mesh(&self, tol: f64) -> Result<Mesh> {
         let mut m = self.tessellate(tol)?;