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/relax/strong.rs

@@ -65,7 +65,7 @@ enum Phase {
 /// The objective's term for one element of quality `q` whose size went from `v0` to `v`.
 fn phi(phase: Phase, q: f64, valid: bool, v: f64, v0: f64) -> f64 {
     if valid && q > 0.0 {
-        let g = if v > 0.0 && v0 > 0.0 { (v / v0).ln() } else { 0.0 };
+        let g = if v > 0.0 && v0 > 0.0 { libm::log(v / v0) } else { 0.0 };
         let shape = match phase {
             Phase::Smooth => (1.0 / q - 1.0).powi(2),
             Phase::MinMax => q.powi(-MIN_MAX_POWER) - 1.0,