feat(docs): split the docs site into per-locale wasm bundles

[vshengbro]

Summary

All locales used to compile into a single wasm bundle (2.24 MB): every visitor
downloaded every language, and the shell resolved locale state at runtime. This
PR splits the build into one bundle per locale, mirroring the directory layout
static i18n sites already use — the default locale at the output root, every
other locale in its own directory (en/, ja/, ko/, …).

build.rs gains the pin: EUV_DOCS_LOCALE selects the locale by prefix,
directory, or label (unset → the default / locale). Alias entries resolve to
their canonical directory; pages of other locales are dropped; the pinned prefix
is stripped from every route and internal link, so in-bundle routes are
prefix-free. Non-default bundles rebase relative asset URLs (./img/… →
../img/…) because assets are copied only by the default build and hosted once
at the site root. The generated code emits SITE_LANGUAGES (switcher entries),
SITE_LOCALE_DIR, and SITE_REDIRECTS next to SITE, and writes
.deploy/locales.tsv so the CLI discovers the remaining locales without a
second YAML parser.

Runtime: the shell keeps rendering from SITE unchanged (it now contains
exactly one locale); the language switcher reads SITE_LANGUAGES and switches
via cross-directory navigation (Location::assign), which the old code already
approximated with a full-page reload. A boot-time redirect before mount maps
legacy single-bundle links (<root>/#/en/…) and alias prefixes (#/zh/…) onto
the right bundle, so old links keep working.

CLI: --locale <name> builds a single bundle for fast iteration; the default
invocation builds the default locale, reads the manifest, then builds every
remaining locale into its own sub-directory. The redundant CLI-side public/
copy is gone (build.rs owns assets now).

Verification

Local build of the 632-page, 4-locale docs site (zh/en/ja/ko):

Bundle	wasm size	Before
zh (root)	984 KB	2.24 MB all-in-one
en	1.02 MB	〃
ja	1.06 MB	〃
ko	1.04 MB	〃

Per-visitor download drops 54–57%. Assets are hosted once at the site root
(total site ~59 MB; naive per-locale asset copies would have been ~214 MB).

Browser-verified on the built output:

• each bundle renders only its own sidebar tree (en bundle's Chinese entries
  are the known untranslated LICENSE page titles, a content gap tracked
  separately, not a routing bug)
• the language switcher preserves the current route across directories
  (/#/ltpp/version.html → /en/#/ltpp/version.html)
• legacy single-bundle links /#/en/ltpp/version.html redirect to the en
  bundle; the #/zh/… legacy alias rewrites in place to #/…
• images load in every bundle (en bundle rebased to ../markdown-images/…,
  11/11 natural-width OK; zh unchanged, 11/11 OK)
• TOC scroll spy still tracks the current section in per-locale bundles
• --locale en produces a single-bundle build with no locale subdirectories

cargo fmt --check clean, clippy --workspace --all-targets 0 warnings, full
workspace test suite passing. Version 0.28.23, workspace pins realigned.

Deploy notes

No workflow change needed in docs-pages/docs: the deploy installs the euv-docs
CLI from master and runs euv-docs docs --out www, which now emits the
per-locale layout (www/en/, …) that cp -r www/* pushes as-is.