fix(unix): avoid Linux-only errno symbol in process liveness check

[ramarivera]

Summary

This fixes a macOS compile/install failure caused by using a Linux-specific libc symbol in the Unix process liveness path.

Problem

is_process_alive used:

• libc::__errno_location()

That symbol is Linux-specific and is unavailable on Darwin, causing install/build failure on macOS with:

• error[E0425]: cannot find function __errno_location in crate libc

Fix

Use a portable errno read on Unix:

• std::io::Error::last_os_error().raw_os_error() == Some(libc::EPERM)

Behavior is unchanged: if kill(pid, 0) fails with EPERM, process is treated as alive.

Validation

Ran locally on macOS (aarch64-apple-darwin):

1. cargo test (25 tests + doc tests): pass
2. cargo install --path . --root <temp>: pass
3. Installed binary executes: mnemoria --version returns mnemoria 0.3.3

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[Copilot]

Pull request overview

This PR fixes a macOS compilation failure by replacing a Linux-specific errno access function with a portable alternative. The is_process_alive function used libc::__errno_location(), which is only available on Linux and caused build failures on macOS with the error "cannot find function __errno_location in crate libc".

Changes:

• Replaced Linux-specific libc::__errno_location() with portable std::io::Error::last_os_error().raw_os_error() for errno checking after kill(pid, 0) system call

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[one-bit]

Hi @ramarivera thank you for your PR. I'll test this on my mac and I'll merge it asap.