mesh: portable atan2 and cbrt for the quality measures (#115)

The platform maths libraries' atan2 and cbrt differ in the last bits
between macOS and glibc, so the dihedral angles and the shape measure a
mesh reports did too. The crate's own versions use + - * / and sqrt
only (correctly rounded everywhere) and agree with the platform's to a
few ulps.

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

crates/mesh/src/tet/measure.rs

@@ -104,7 +104,7 @@ pub fn dihedral_range(x: &[P; 4]) -> (f64, f64) {
     let mut lo = f64::INFINITY;
     let mut hi = f64::NEG_INFINITY;
     for (sv, cv) in s.iter().zip(c) {
-        let a = sv.abs().atan2(cv).to_degrees();
+        let a = crate::vec3::atan2(sv.abs(), cv).to_degrees();
         lo = lo.min(a);
         hi = hi.max(a);
     }