Deterministic IPv6 external-IP discovery #220

Background

Surfaced in PR #212 review by @chatgpt-codex-connector (resolver.py:43 thread):

On a dual-stack host this new IPv6-capable service is still selected through _pop_random_ip_host(), so most runs can hit one of the IPv4-only endpoints and store the IPv4 external address. Since Judge.check() later accepts a judge only when that single real_ext_ip appears in the page, any judge reached over IPv6 will be rejected even though IPv6 works.

Why this needs its own ticket

Adding api64.ipify.org to _ip_hosts (PR #212) was a necessary first step — the resolver no longer rejects IPv6 responses. But _pop_random_ip_host() still picks uniformly at random, so on a dual-stack host you get IPv4 ext-IP about 80% of the time (4 of 5 default hosts are v4-only).

The right fix is a design choice with at least three reasonable options:

1. Prefer api64.ipify.org first (deterministic). Try it; only fall back to others if it fails. Simplest, but loses load-balancing across services.
2. Probe both v4 and v6 separately and store both real_ext_ip_v4 + real_ext_ip_v6. Judge.check() then accepts whichever appears on the page. Most correct for dual-stack hosts but requires plumbing two values through the comparison sites.
3. Happy Eyeballs at the IP-discovery layer (race the v4 and v6 probes in parallel, take the first to succeed). Mirrors the DNS layer behavior added in PR feat(ipv6): full IPv6 support across the stack (#201) #212.

Each has different trade-offs (latency, cost, code complexity, observability). PR #212's scope was 'IPv6 first-class across the stack' — this is a follow-up refinement.

Acceptance criteria

• A user on a dual-stack host running Resolver().get_real_ext_ip() reliably gets an address whose family matches the network the broker will then connect to judges over.
• If only one family is reachable, that one wins (no false-negative rejections in Judge.check()).
• Existing IPv4-only contracts preserved (caller passing family=AF_INET or similar should be respected if/when introduced).