- 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.