The Docusaurus build step (198s) is 79% of the docs-site-checks job
wall time and is the CI critical path. The site has two locales (en +
zh-Hans, ~700 pages total); building only the default locale in PR
checks halves the build time.
Follows the same pattern Docusaurus uses internally: a build:fast
script that runs `docusaurus build --locale en` for CI/preview builds,
while the full bilingual build runs only in deploy-site.yml for
production deploys.
deploy-site.yml is unchanged — it still runs `npm run build` (all
locales) on push-to-main and release.
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