tcp_user_timeout 0.1.0

Wraps the Linux TCP_USER_TIMEOUT (and optional SO_KEEPALIVE) socket options behind a fiber-safe block API. Sockets opened inside the block inherit a deadline the kernel itself enforces — Ruby threads parked in blocking syscalls that Thread#kill and Timeout.timeout cannot interrupt are released when the kernel drops the connection. No-op on macOS and other non-Linux platforms.