* fix(validate-sidebar): replace subdirectory heuristic with locale pattern matching
detectLanguageDirs() previously classified any top-level docs/ directory
containing subdirectories as a translation language. This was too broad —
if an English section ever gained nested subfolders it would be silently
excluded from validation.
Replaced with a BCP 47 locale-code regex (/^[a-z]{2}(?:-[a-zA-Z]{2})?$/)
that matches known patterns (cs, fr, vi-vn, zh-cn) and won't falsely
classify content sections like explanation/ or reference/.
* fix(validate-sidebar): guard drift check against undefined order values
extractSidebarOrder() returns { hasSidebar: false } when no sidebar block
exists, leaving order as undefined rather than null. The drift check only
guarded against null, allowing undefined values to emit noisy warnings
like "Order drift: ... order undefined".
Changed the guard to typeof === 'number' which correctly excludes both
undefined and null without relying on a specific sentinel value.
* chore(validate-sidebar): add JSDoc docstrings to all functions
Adds @param and @returns annotations to extractSidebarOrder,
detectLanguageDirs, getEnglishSections, checkDirectory,
checkTranslationDrift, and relativePath.
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