docs(i18n): document complete Docsy locale support

[Vonng]

Documents pgsty/oink#39 and pgsty/oink#38.

What changed

• Adds the canonical EN/ZH interface-localization contract: the exact 31 Docsy-compatible locale names, OINK's generic zh, the 192-key schema, placeholder and scalar rules, reviewed shared terms, and RTL direction ownership.
• Documents the Hugo 0.160.x configuration edge: a non-default generic zh beside regional Chinese catalogs uses locale: zh-CN; bare locale: zh works in this configuration from Hugo 0.161.
• Updates the public Languages and Features pages to remove the old claim that OINK-only labels fall back to English.
• Updates the comparison table to distinguish OINK's 32 complete catalogs from Docsy's current 31 locale files.
• Replaces deprecated languages.<lang>.languageDirection examples with Hugo's current languages.<lang>.direction key in the active EN/ZH guides. Dated research remains unchanged as historical evidence.
• Refreshes the single affected llms.txt golden.

Review and verification

Claude Code Fable 5 at xhigh effort gave the final cross-repository diff a PASS with no P0/P1/P2 finding remaining after three adversarial passes.

Validated with HUGO_MODULE_REPLACEMENTS='github.com/pgsty/oink -> /Users/vonng/pgsty/oink':

• full npm test;
• Markdown style;
• 128/128 bilingual pages and 977 aligned headings;
• 318 rendered pages, 27,829 internal links, and 3,408 fragments;
• all 15 Markdown-output goldens;
• Playwright: 29 WCAG + 45 responsive/palette + 16 keyboard + 10 content + 18 code-block + 4 PRD5 + 5 theme-colour = 127 tests.

Integration and release state

The maintainer explicitly promoted this exact green documentation head to main on 2026-09-03 after the theme source integration. It documents implemented but unreleased behavior; it does not claim that a public module tag or consumer pin already contains the feature.

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