find_node_ip() returns None on loopback-only hosts, which is not a valid MASTER_ADDR #302

find_node_ip() ends with return main_ip or virtual_ip, so it returns None when no interface other than loopback exposes an IPv4 address (for example a process started with docker run --network none, or a container whose only NIC is not up yet).

RayHelper stores the result into MASTER_ADDR:

env_vars['MASTER_ADDR'] = ip

None is not a valid environment variable value, so instead of a clear "could not determine node address" the job fails inside Ray's env_vars validation.

Reproduction (pure Python, no Ray needed) — load src/twinkle/utils/network.py, stub psutil.net_if_addrs() to return only lo:

find_node_ip() -> None
MASTER_ADDR    -> None   # invalid env var value

Proposed fix: keep the current preference order (non-virtual interface, then virtual interface) but make the final fallback 127.0.0.1 instead of None.

I have a PR with the fix plus a regression test: #301