mesh: platform-independent maths (libm) so meshes are bit-identical on macOS and Linux

The system maths library's sin, cos, atan2, log … are not correctly rounded and differ
between platforms: mesh.hex.generate gave the plate of the strong-relax test a different
mesh on Linux than on macOS (nodes from 2809 on), so the test passed on a Mac and failed
in CI. Every transcendental call in fieldcraft-mesh now goes through the libm crate (a
pure-Rust port of musl's), which gives the same bits everywhere.

Part of #438.

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

crates/mesh/src/planar.rs

@@ -266,7 +266,7 @@ pub fn face_boundary(positions: &[[f64; 3]], triangles: &[[u32; 3]], tri_face: &
             .map(|i| {
                 let (p, q, r) = (at(i + n - 1), at(i), at(i + 1));
                 let (u, w) = ([q[0] - p[0], q[1] - p[1]], [r[0] - q[0], r[1] - q[1]]);
-                let turn = (u[0] * w[1] - u[1] * w[0]).atan2(u[0] * w[0] + u[1] * w[1]).abs();
+                let turn = crate::vec3::atan2(u[0] * w[1] - u[1] * w[0], u[0] * w[0] + u[1] * w[1]).abs();
                 ids[i] != ids[(i + n - 1) % n] || turn > CORNER_ANGLE
             })
             .collect();