fix: cache translation lookups to ensure hreflang reciprocity

[pettinarip]

Summary

• Cache getTranslatedLocales() results per slug in translationRegistry.ts
• Cache areNamespacesTranslated() filesystem checks per locale+namespace in translationStatus.ts

Context

The March 2026 SEO audit flagged 97 pages with broken hreflang reciprocity (943 flagged rows). Google requires that if Page A declares Page B as an alternate-language version, Page B must declare Page A back.

Root cause: getTranslatedLocales() and areNamespacesTranslated() use uncached existsSync() calls. During a Next.js build, different pages render at different times. If translation files shift mid-build (e.g., concurrent Crowdin sync), different pages can see different filesystem states for the same slug — producing inconsistent locale lists that break hreflang reciprocity.

Why it's intermittent: We checked all 10 flagged route+locale combinations against the current production deploy and found all hreflang sets consistent. The issue only manifests when the filesystem changes during a build.

Fix: Module-level Map caches (same pattern as the existing cachedStaticPages in staticPages.ts) ensure that once a slug's translated locales are computed, every subsequent caller gets the same result for the lifetime of the build process.

Side benefit: Eliminates redundant existsSync() calls (25 locales × ~5,000 pages = up to 125K filesystem reads reduced to ~5K).

Test plan

• TypeScript passes (npx tsc --noEmit)
• Build completes successfully (pnpm build)
• Verify hreflang tags are consistent across locales for a sample page

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[myelinated-wackerow]

Clean, well-scoped fix. Memoization logic is correct, cache keys are sound, no callers mutate the returned arrays, and the pattern matches existing cachedStaticPages convention. Approve.

Minor observations (non-blocking):

1. Dev server staleness — Module-level caches persist for the process lifetime, so adding/removing translation files during next dev requires a restart to take effect. Matches existing behavior of cachedStaticPages.

2. Multi-worker builds — If Next.js spawns multiple workers during build, each gets its own cache. Two workers could theoretically see different filesystem states, though the window is much narrower than the current uncached approach.

3. Defensive copy — Current callers are safe, but returning [...cached] instead of the reference directly would guard against future callers accidentally mutating the cache.

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Reviewed by Claude (Opus 4.6)