Emit hreflang x-default when more than one translation exists

[apermo]

Problem

MslsOutput::get_alternate_links() only outputs an hreflang="x-default" alternate when
exactly one linked translation exists:

if ( 1 === count( $arr ) ) {
    return apply_filters( self::MSLS_ALTERNATE_LINKS_DEFAULT_HOOK, $default );
}

$arr = (array) apply_filters( self::MSLS_ALTERNATE_LINKS_ARR_HOOK, $arr );

return implode( PHP_EOL, $arr );

When two or more translations exist — the normal multilingual case — x-default is dropped and
only the language-specific alternates are returned.

x-default is the fallback search engines use when no other hreflang matches the visitor's
locale (typically a language selector or the default-region page). Google and the W3C recommend
including it for multilingual pages, so omitting it precisely when several languages exist is the
opposite of the recommended behaviour.

• Google Search Central — Localized versions: https://developers.google.com/search/docs/specialized/international/localized-versions
• Website specification — hreflang: https://specification.website/spec/i18n/hreflang/

Proposed fix

In the multi-alternate branch, also emit the already-computed $default (x-default) link, run
through the existing msls_output_get_alternate_links_default filter so it stays customisable
and can be suppressed by returning an empty string. Additive and backward compatible.

Acceptance criteria

• A page with ≥2 translations outputs an hreflang="x-default" alternate alongside the
  language-specific ones.
• Returning '' from msls_output_get_alternate_links_default suppresses x-default.
• Single-translation behaviour is unchanged.