i18n: generate zh-Hant from zh-CN

zh-Hant is the zh-CN text in Traditional characters, but contributors
still translated it by hand from `--missing zh-Hant`, and a model asked
for Traditional Chinese writes Taiwan vocabulary (影片, 伺服器, 載入).

`npm run i18n -- --zh-hant` now rewrites zh-Hant.json from zh-CN.json
with OpenCC s2tw (opencc-js 1.4.2, cn -> tw, characters only), then the
fixed list of characters it picks wrongly. The check that `npm test`
runs fails when zh-Hant differs from that conversion, naming the key,
and `--missing zh-Hant` points to the command instead of printing a
template.

The conversion reproduces every one of the 3346 entries on main. 25
keys were out of zh-CN order and move; no value changes.

The converter loads on first use, so a copy of the script without
node_modules (stale-index.test.mjs) still extracts messages.

docs/CURRENT-STATE.md

@@ -5096,7 +5096,9 @@ Shipped locales: **en** (source and fallback), **ar**, **de**, **es**, **fr**, *
 - **Gate.** `npm run i18n` (and `src/i18n/catalog.test.mjs` under `npm test`)
   extracts every message and fails on missing, empty, unused,
   placeholder-mismatched, still-English (plural forms included) or
-  wrongly-shaped plural entries.
+  wrongly-shaped plural entries. `zh-Hant` is generated from `zh-CN`
+  (`npm run i18n -- --zh-hant`: OpenCC `s2tw` through `opencc-js`, then a
+  fixed list of character corrections), and the gate fails when it differs.
 - **Not translated by design:** layer ids, share-link and hash parameters, API
   fields, voice tool schemas, provider names, feed data (callsigns, vessel and
   station names) and attribution.