catena v1.3.0

Documentation and site only — no change to the library's API or behaviour.
The headline is that the docs are now versioned: a reader pinned to an
older release can read that release's documentation instead of the current
one silently replacing it.

Added

• Versioned documentation. The site serves the current release at the
  root and archived releases under their own prefix, with a switcher in
  the header. v1.1.0 is archived at /1-1/, and its pages carry a notice
  saying so. The snapshot is taken from the tag rather than the working
  tree — the versioning plugin archives whatever is currently checked out,
  which would have filed the newer API under the older version's name.
• A 1280×640 social preview card, wired as og:image and twitter:image
  across every page. Links to the site previously unfurled with no image
  at all. The same file is the repository's GitHub social preview, which
  has no API and is uploaded by hand.

Changed

• The documentation site is now published from released tags only. Its
  content is derived from the tree it is built from while its version
  label comes from the newest release, so publishing main put the two in
  disagreement the moment anything landed unreleased — the site briefly
  documented BottomN while announcing itself as v1.1.0, which does not
  have it. A push to main now builds the site and runs every check
  against it without deploying. A site-only change can still be published
  from a branch, guarded by a check that refuses unless docs/operators/
  matches the latest release tag.
• Canonical URLs, the sitemap and the og:image now name
  catena.nerdmenot.in. The site answers on both that and its Pages
  subdomain, and naming the subdomain made the branded domain the
  duplicate.
• The version switcher is the site's own component rather than the
  plugin's native <select>, whose option list is drawn by the operating
  system and takes none of the site's type or colour. Versions are links,
  so one can be opened in a new tab.
• API and GitHub in the top navigation open in a new tab, on the
  landing page and in the docs header.
• The README links the documentation site, which it never had.

Fixed

• The theme toggle lost its selected state on documentation pages. It
  renders twice there — the header and the mobile menu — and both named
  their radios catena-theme; radios sharing a name form one group however
  far apart they sit, so the browser kept only the last checked one.
• Switcher links pointed at pages that need not exist in every version.
  The 404 page's switcher offered /404/ and /1-1/404/, neither a
  route. Links now fall back to a version's front page where the same page
  is absent, and /1-1/ itself is a page rather than a 404.
• scripts/sync.ts cleared all of src/content/docs/ on every run, which
  would have destroyed the archived versions permanently — nothing can
  regenerate a snapshot of a tree that no longer exists. It now clears only
  the directories it generates.

Removed

• SPEC.md. The design document had become a second place where the
  library's contracts were written down, and the two could disagree — as
  they did: its Try rule set contradicted the code, its symbol counts had
  drifted, and it specified a conformance harness shape that does not
  exist. Each operator's contract lives in its own doc comment, where godoc
  shows it and the conformance suite enforces it. The document remains in
  the v1.2.0 tag for anyone who wants the rejected-alternatives history.