Daemonize heartbeat thread

[kojix2]

• IRuby’s heartbeat uses ZMQ::Device (zmq_proxy) which blocks indefinitely.
• On Ruby 3.4, a non-daemon blocking thread prevents process exit.
• Mark the heartbeat thread as daemon to avoid exit hang while keeping behavior unchanged.

As a step towards fixing #137

[kojix2]

Hmm. It seems the API for daemonising Ruby threads only exists in JRuby.
I don't think the direction of the fix is wrong, but this is definitely an AI hallucination.

[kojix2]

#357 (comment)