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

Cargo.lock

@@ -1418,6 +1418,7 @@ name = "fieldcraft-mesh"
 version = "0.6.0"
 dependencies = [
  "fieldcraft-model",
+ "libm",
  "num-bigint",
  "num-rational",
  "num-traits",