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).

An enabled language that has NO domain of its own keeps its locale prefix and lives on the primary domain, so its URLs are published from the primary domain's file (they are already what this module's alternates point at). Mapped hosts stay strictly single-language.

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.