- mtglib/proxy.go: rename private field domainFrontingIP -> domainFrontingHost
  and update DomainFrontingAddress() doc comment to reflect that hostnames
  are now accepted. The exported mtglib.ProxyOpts.DomainFrontingIP is
  unchanged (public API), so the assignment in NewProxy now reads
  `domainFrontingHost: opts.DomainFrontingIP,` which makes the
  public-vs-internal naming explicitly visible at the boundary.
- internal/config/{parse,config}.go: reorder so Host comes before IP in
  the [domain-fronting] struct. Cosmetic, but signals Host is the
  preferred forward path.
- Add TestDomainFrontingHostAcceptsLiteralIP + domain_fronting_host_ip.toml
  fixture exercising the documented "host accepts hostname or literal IP"
  contract end-to-end.

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).