Multi-domain sitemap post-processing (one domain = one canonical language).

Activated by a host-app provider:

config :phoenix_kit, :sitemap_domains_provider, {MyAppWeb.SEO, :sitemap_domains}

The provider returns [%{host: "example.fr", language: "fr", primary: false}, ...]. The result is VALIDATED here — exactly one primary: true, every host a distinct language (base-code level), well-formed hosts. Any violation deactivates domain mode (with a warning) rather than silently producing malformed URLs or dropping x-default.

rebuild_for_domains/2 regroups the entries the generator already collected across languages: for the domain of language L, every canonical group that HAS an L entry contributes one URL — L's own entry re-hosted on that domain with its locale prefix stripped (L is the default there) — carrying an alternates set computed ONCE per group (identical across all domain files by construction): each group language on its home URL, plus x-default for the primary domain's language when the group has it (omitted otherwise — never mislabel another language as x-default).

Runs inside Oban jobs and unpiped controller requests: language resolution here never consults the request-scoped default-language override — the unprefixed-entry fallback reads the raw is_default flag, mirroring the generator's hardened get_languages/0.

NB: the generator's base_url (Settings site_url) and the provider's primary host are independently-edited settings that must refer to the same site; rel-path extraction below is host-string-agnostic, so a mismatch yields consistent (if unexpected-host) URLs rather than corruption.