Fix missing libm linkage for _core extension on Linux

[Siyet]

Summary

• Add explicit -lm linkage for the _core C extension on non-Windows platforms
• Fixes undefined symbol errors (nextafter, fmod, lround) on aarch64 Linux systems where libm is not implicitly linked

Closes #971

Test plan

Built and verified with ldd on two environments:

Ubuntu 22.04 (WSL2, x86_64, gcc 11.4, Python 3.10):

	libm linked?
Without fix	No - only libc.so.6
With fix	Yes - libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6

Ubuntu 24.04 (WSL2, x86_64, gcc 13.3, Python 3.12):

	libm linked?
Without fix	No - only libc.so.6
With fix	Yes - libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6

Windows 11 (Python 3.12, AMD64):

Confirmed that -lm is not added on win32 - libraries list stays empty. libm is part of MSVC runtime, no explicit linkage needed.

• CI passes on all platforms

[Siyet]

CI failures are unrelated to this change:

• "Profile Windows (ARM64)" - runner infra issue ("Install command runner" step)
• "build" (documentation) - link checker fails on a pre-existing broken link (Broken link to ravyn.dev encoders example in docs #976)

All build, test, and wheel jobs pass across all platforms.