Localization: wire AI plural translation into the GlotPress reverse fold

[jkmassel]

Wires the AI translation primitives from #25705 into the plural reverse fold — the live tier behind the human ?? AI ?? English floor — filling the ai_translate_plural stub #25688 left open.

Stacked on #25705 (its primitives aren't on trunk yet); the diff here is wiring + tests + docs only, and the base retargets to trunk once #25705 merges. Draft to match #25705 and because the live fold wants a real download run to validate.

Scope: plurals only

Plurals fold into Plurals.xcstrings — a String Catalog with a needs_review state — which is built into the app but not consumed at runtime yet (no code reads it; plurals still render the legacy way). So the AI tier pre-populates the catalog for the plural cutover without shipping any machine-translated copy today.

Regular strings are deliberately out of scope: they still ship from the legacy Localizable.strings, where a machine translation would go live immediately — and we don't want machine-translated copy shipping before the catalog cutover. Regular-string MT waits for a catalog reverse-fold (same shape as plurals), where it can stage as needs_review until cutover. The rationale is captured in docs/localization-pipeline.md so it doesn't need re-explaining.

Summary

• download_localized_plurals now drives AITranslator instead of the nil stub — gated on ANTHROPIC_API_KEY (absent ⇒ unchanged English-fallback behavior) and run as a non-fatal step, so the AI tier can never break a release.
• The reverse fold calls the model once per (key, locale) form-set (translate_plural), with the human-translated forms as anchors — one consistent stem across the forms, not per-cell drift.
• New pure, stdlib-only plural_strings_helper_test.rb (picked up by Localization: AI translation primitives #25705's existing Localization Tooling Unit Tests CI job); nokogiri made lazy so the suite needs no gems.
• docs/localization-pipeline.md documents the GlotPress + AI round trip end to end and why regular-string MT waits for the catalog.

Form-set, not per-cell

PluralStrings.fold_translations!'s ai_translator contract changed from per-cell to a per-(key, locale) form-set callable:

ai_translator.call(english_forms:, categories:, locale:, note:, anchors:) => { <cat> => translation }

AITranslator#translate_plural implements it directly — ai_translator: translator.method(:translate_plural). This is deliberately not the per-cell for_plural adapter #25705's notes floated: translating the whole set in one request is the fix for the synonym drift a per-category call structurally can't prevent (Polish słowo → wyrazy → słów). A per-set API failure degrades to English (needs_review) rather than aborting the fold.

Test plan

• ruby fastlane/lanes/plural_strings_helper_test.rb — 10 (fold provenance + the form-set/anchors contract)
• Full fastlane/lanes/*_test.rb suite green — 60 runs
• rubocop clean on all changed files
• Live plural fold against the API (needs ANTHROPIC_API_KEY + bundle install) on a real download — confirm quality before the plural cutover

Related

• Localization: AI translation primitives #25705 — AI translation primitives (this PR's base; this wires them in)
• Localization: String Catalog pipeline (plurals + catalog generation) #25688 — String Catalog pipeline (left the ai_translate_plural stub this fills)

[dangermattic]

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