Use std::chrono features in the UNIX SocketMonitor class

Mirrors the fix from PR #707 for the Windows implementation: replaces
clock()/clock_t with std::chrono::steady_clock for monotonic, high-resolution
timing that avoids clock() rollover and CLOCKS_PER_SEC inaccuracies.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>