Env server fails to start on Windows: ZMQ ipc:// transport is unsupported

[martian56]

What happens

Running an evaluation on native Windows (prime eval run ..., which spins up the
verifiers env server) fails before a single rollout runs. The env-server subprocess
can't bind its internal ZMQ sockets, and the client receive loop errors out too.

Two errors fire:

1. Client receive loop:

   Proactor event loop does not implement add_reader family of methods required for
   zmq. zmq will work with proactor if tornado >= 6.1 can be found. Use
   asyncio.set_event_loop_policy(WindowsSelectorEventLoopPolicy()) or install
   'tornado>=6.1' to avoid this error.

2. Env server (fatal):

   zmq.error.ZMQError: Protocol not supported (addr='ipc:///tmp/vf-...-responses')

Root cause

verifiers/utils/serve_utils.py:

def make_ipc_address(session_id, name):
    return f"ipc:///tmp/vf-{session_id}-{name.replace('/', '--')}"

The router↔worker sockets use ZMQ ipc:// with a hardcoded /tmp path. ipc:// is
Unix-domain-socket only and isn't supported on Windows, so EnvRouter.__init__ fails at
bind(). There's no tcp fallback or override. Installing tornado only silences the
first Proactor error; the ipc:// bind still fails regardless of the event loop.

Impact

Anything that starts the env server — prime eval run, prime gepa, local prime rl —
cannot run on native Windows. Env authoring, env install, and env push are unaffected.

Environment

• OS: Windows 11
• Python 3.12
• verifiers 0.2.0 (same code on main)

Steps to reproduce

prime env init demo        # any trivial SingleTurnEnv
prime env install demo
prime eval run demo -m openai/gpt-4o-mini -p openrouter -n 1 -c 1 --disable-tui

Traceback (paths trimmed):

verifiers.serve.client.env_client.ZMQEnvClient - ERROR - Unexpected error in receive loop
for env server demo (tcp://127.0.0.1:PORT) (Proactor event loop does not implement
add_reader family of methods required for zmq. zmq will work with proactor if tornado
>= 6.1 can be found. Use `asyncio.set_event_loop_policy(WindowsSelectorEventLoopPolicy())`
or install 'tornado>=6.1' to avoid this error.)
Traceback (most recent call last):
  File ".../zmq/asyncio.py", line 51, in _get_selector_windows
    from tornado.platform.asyncio import AddThreadSelectorEventLoop
ModuleNotFoundError: No module named 'tornado'

During handling of the above exception, another exception occurred:

  File ".../verifiers/serve/client/zmq_env_client.py", line 198, in receive_loop
    msg = await self.socket.recv_multipart()
  ...
RuntimeError: Proactor event loop does not implement add_reader family of methods required
for zmq. ...

Process SpawnProcess-1:
Traceback (most recent call last):
  File ".../verifiers/serve/server/env_server.py", line 145, in run_server
    server = cls(*args, **kwargs)
  File ".../verifiers/serve/server/env_router.py", line 134, in __init__
    self.response_pull.bind(self.response_address)
  File ".../zmq/sugar/socket.py", line 320, in bind
    super().bind(addr)
  ...
zmq.error.ZMQError: Protocol not supported (addr='ipc:///tmp/vf-...-responses')

Workaround

Run under WSL/Linux, where ipc:///tmp works.