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/Cargo.toml

@@ -10,6 +10,10 @@ workspace = true
 
 [dependencies]
 fieldcraft-model = { workspace = true }
+# Pure-Rust maths (a port of musl's libm) for sin, cos, log, exp, pow, hypot, acos: the same
+# bits on every platform, where the system library's are not correctly rounded and differ (the
+# plate's hex mesh differed between macOS and Linux, #438). atan2 and cbrt are crate::vec3's.
+libm = "0.2"
 thiserror = { workspace = true }
 
 [dev-dependencies]