🐛 Leave an alternate that is not a language variant on --base

[taras]

Motivation

rel=alternate covers two unrelated things, and #22 treated them as one.

With hreflang, it is this page in another language:

<link rel="alternate" href="…" hreflang="en">

That is a claim about the page, so it belongs with the canonical url.

Without it, it points at a separate file:

<link rel="alternate" type="application/rss+xml" href="…/feed.xml">
<link rel="alternate" type="text/plain" href="…/llms.txt">

A file lives where the build is served. Sending those to --canonical would
publish a feed url on a site that does not host the feed.

Nothing has hit this. Effection writes both of those relatively, and staticalize
only rewrites urls that start with the crawl origin, so it never touched them —
I noticed it while reading the output of thefrontside/effection#1255 rather than
from a failure.

Approach

names() splits the two cases. hreflang is the whole test, since that is what
distinguishes a translation of this page from a different representation of it:

if (rel.includes("alternate")) {
  return Boolean(element.properties?.hrefLang);
}

hrefLang is how hast spells the attribute; I checked rather than assumed.

Tests

Extended the existing split test with a feed alternate, so one case now asserts
both halves of the rule: alternate[hreflang] lands on --canonical, and
alternate[type=application/rss+xml] on --base.

I confirmed the assertion actually catches the old behaviour by reverting
names() and re-running:

sends urls that name the page to --canonical, and the rest to --base ... FAILED

'<link rel="alternate" type="application/rss+xml" href="https://frontside.com/interactors/feed.xml">'
  doesn't contain the expected item
'<link rel="alternate" type="application/rss+xml" href="https://interactors.netlify.app/feed.xml">'

All 33 steps pass with the fix.