Follow-up to the previous commit on this branch:

- Rename Config.GetDomainFrontingIP -> GetDomainFrontingHost. The
  helper now returns a hostname or an IP, so the old name was a lie.
  Drop the unused defaultValue net.IP parameter (every caller passed
  nil). Update internal/cli/run_proxy.go and internal/cli/doctor.go;
  rename the misleading `ip` local var in doctor.go to `override`.

- Add TOML fixtures (domain_fronting_host.toml, domain_fronting_ip.toml)
  so the new field is exercised through the actual Parse()->JSON->Config
  path users hit, not just via direct .Set() calls. Plus a positive
  backward-compat test confirming an `ip`-only legacy config still
  validates and resolves correctly, and a no-fronting test confirming
  the unset case returns empty.

- Clarify example.config.toml: `ip` is kept for backward compatibility,
  not because it has stricter validation semantics worth choosing over
  `host`.

mtglib.ProxyOpts.DomainFrontingIP keeps its name (public API).