Modernize core error flow and netlink parsing

[fakedude]

Summary

• align core operational error paths with std::expected and add deterministic tests for socket/control failure propagation
• remove 0U literals in modernization scope and preserve bitshift style rules
• add checked netlink parsing helpers in nl_common and adopt shared nlmsgerr parsing in listener/route/link/address task handlers

Test Plan

• cmake --build build -j
• ctest --test-dir build --output-on-failure