docs(i18n): consolidate three i18n docs into one + refresh README + PERFORMANCE

[abdout]

Severity

P3 — Documentation hygiene

Context

Three overlapping i18n docs disagreed about reality:

• `INTERNATIONALIZATION.md` referenced legacy paths (`[locale]` segment, `lib/locales/`, `middleware.ts`)
• `INTERNATIONALIZATION_GUIDE.md` was a 23 KB Databayt-wide standard duplicated into this repo (belongs in `kun`)
• `README_I18N.md` was a quick-start with `localhost:3001` and stale Next.js 15 references

`README.md` was the unedited `create-next-app` boilerplate. `PERFORMANCE.md` referred to "Next.js 15".

Fix

• Delete `INTERNATIONALIZATION_GUIDE.md` (org-wide standard belongs in kun, not marketing)
• Delete `README_I18N.md` (overlap)
• Rewrite `INTERNATIONALIZATION.md` as the single source of truth, accurate to today's setup: `[lang]` segment, `proxy.ts` (Next 16), eager-loaded client dicts, locale-aware formatters, RTL logical-prop policy
• Replace `README.md` with a real project description (stack, routing, env vars, doc links)
• Update `PERFORMANCE.md` Next.js 15 → 16 reference

Acceptance criteria

• Repo grep for `Next.js 15` (in docs) returns nothing
• `INTERNATIONALIZATION.md` reflects `proxy.ts` and `[lang]` (not `middleware.ts` / `[locale]`)
• README points new readers to the correct dev commands and env vars

Reference

Plan: PR-19.