Sticky ?lang= navigation + site-wide hreflang/canonical tags

- Server replicates a ?lang= override onto the navigation links it
  renders (nav, sidebar, cards, brand), so clicks and prefetches stay in
  the chosen language even without JS; link_lang is part of the ETag and
  body cache key (query and header renders of the same language differ
  in their links).
- pagerite.js drops the Accept-Language header hack: the remembered
  language rides internal fetches as ?lang= instead (added when a link
  lacks one), the page cache keys on path+query, and history/address bar
  keep the pretty query-less URL.
- Canonical names the actually served language (plain URL for the
  original, ?lang= for translations); hreflang alternates are site-wide
  from translate_langs, identical on every page: x-default (the plain
  autodetecting URL) first, then every language explicitly, default
  included, emitted right after canonical before the social tags.

docs/migrate.md

@@ -119,8 +119,9 @@ translation data, in the same transaction:
   falling back to `chunks[h]`; then apply `patches.get(f"{path}:{L}", [])`
   in order (per-hunk, best effort); then `markdown.render` as today. All of
   this assembles the `Translation` the phase-1 plumbing already consumes.
-- **Availability:** `available_languages(path)` = `sorted(node.langs)`;
-  hreflang alternates and `?lang=` handling use exactly this set.
+- **Availability:** `node.langs` is the availability index; `?lang=`
+  handling uses exactly this set. (hreflang alternates are site-wide from
+  `translate_langs` instead — see docs/localization.md.)
 - **Save (primary language):** server re-chunks the submitted Markdown,
   inserts new hashes into `Data.chunks`, replaces `node.chunks`. Unchanged
   chunks keep their hashes — only genuinely new text lands in the diff.