✨ Separate where a build is hosted from the url it is canonically at #21

Motivation

--base currently answers two different questions at once:

1. where these bytes live — what a[href], [src], and asset links should
   point at
2. what this content is called — what <link rel="canonical"> and
   <meta property="og:url"> should claim

For an ordinary site those coincide, so nothing forced them apart. They diverge
the moment a site is served from one origin and published at another, which is
how the Frontside sites work: Effection is hosted at effection.netlify.app and
published at frontside.com/effection, and the same holds for Interactors and
Graphgen.

Today the only way to get a correct canonical is to point --base at the
published url, which then drags every other url along with it. The build becomes
valid only at the published address, and a preview cannot say "index production
instead of me" without also rewriting its own navigation onto production.

It has already cost something

thefrontside/effection#1248 moved Effection's production --base to
https://frontside.com/effection to get the canonical right. That also moved the
sitemap, so effection.netlify.app/sitemap.xml began listing
https://frontside.com/effection/.... frontside.com mounts a proxied site's
sitemap by prefixing /effection onto every entry, so the prefix landed twice
and every url 404'd:

Error: could not download http://127.0.0.1:8005/effection/effection/docs/resources
  Error: GET ... responded 404 Not Found

519 errors, and frontside.com's deploy has been failing since
(thefrontside/frontside.com#509 is the fix on that side). None of that was the
intent of #1248 — it followed from --base meaning two things.

What is actually being rewritten

Worth recording, because it narrows the problem. On the deployed builds, no
navigation was rewritten at all:

Every one of those 6 and 4 is identity metadata:

<link rel="canonical">      <meta property="og:url">
<meta property="og:image">  <meta name="twitter:image">
<link rel="alternate" hreflang=…>

Both sites navigate with relative hrefs, so they still browse standalone. The
conflation is not breaking navigation today — it works by luck rather than by
design, and it did break the sitemap.

Approach

Add --canonical, defaulting to --base, so every existing invocation behaves
exactly as it does now. rebase() then gets called with one of two targets
depending on what the url is for:

A preview then expresses what it actually means:

--base=https://pr-333--interactors.netlify.app   # browsable on its own
--canonical=https://frontside.com/interactors    # index production, not me

Open questions

og:image / twitter:image. Assets, so base by the rule above — a
preview shows its own card. But a social card arguably should always be
production's. Listed as base in the table; easy to move.

The sitemap. It maps this deployment, so base keeps a preview from
listing production urls, and keeps frontside.com's prefixing working as it
always did. Google's guidance is that sitemaps list canonical urls, which argues
the other way.

Textual bodies (#17). The sharp one. #1248's intent was that production
documents advertise production, which it got by pointing --base there. Split
canonical out, leave text on base, and llms.txt goes back to naming the
Netlify origin — a regression of that intent. canonical seems right, since
llms.txt is a map telling agents where the docs live. The exception is a
feed's rel="self", which should name where the feed is, and a blind text
substitution cannot tell the two apart.

What the consuming sites do afterwards

• Interactors — flag change only. It already emits its canonical from the
  crawl origin, which is what lets staticalize rewrite it.
• Graphgen — needs the <link rel="canonical"> it never had
  (🐛 Specify canonical url as frontside.com/graphgen with staticalize graphgen#74), then the same flag change.
• Effection — gets simpler. Its canonical is currently emitted by the site
  from its own base option, so staticalize skips it as off-origin. If
  staticalize owns canonical, that plumbing collapses and Effection stops having
  two different flags named --base.