Use macOS boot session API for liveness

[yvette-carlisle]

Summary

• read macOS host boot identity through kern.bootsessionuuid via sysctlbyname
• remove the sysctl subprocess and kern.boottime timeval dependency from liveness markers
• add a regression test for the boot-session UUID marker prefix

Verification

• cargo test -p decodex macos_host_boot_id_uses_boot_session_uuid
• cargo test -p decodex live_process
• cargo test -p decodex previous_boot
• cargo make fmt-check
• cargo make check-rust