fix(build): make messenger, ui and mdx dist resolvable by Node's ESM loader

[laruss]

What was broken

messenger, ui and mdx declare "type": "module" with only an exports.import condition, so Node reads their published output as ESM — where relative specifiers need an explicit file extension and a directory never resolves to its index.js. Their builds were missing tsc-alias's resolveFullPaths, which core and devtools already set, so dist shipped the shortened forms that only bundlers and CJS accept:

import "./i18n";                    // ERR_UNSUPPORTED_DIR_IMPORT
import { logger } from "./logger";  // ERR_MODULE_NOT_FOUND

Reproducible in two lines against the previously published tarball:

cd node_modules/@react-text-game/messenger/dist
node -e "import('./index.js').catch(e=>console.log(e.code))"   # ERR_UNSUPPORTED_DIR_IMPORT
node -e "import('./chat.js').catch(e=>console.log(e.code))"    # ERR_MODULE_NOT_FOUND

mdx had the same missing setting and is fixed alongside.

The second, quieter bug in ui

resolveFullPaths does not fix @react-text-game/ui/i18n: it imported its English strings from JSON, which Node refuses without a with { type: "json" } import attribute (ERR_IMPORT_ATTRIBUTE_MISSING).

This one failed silently rather than loudly. core reaches that entry through a dynamic import inside Game.init, wrapped in a try/catch that treats any failure as "the UI package isn't installed" (packages/core/src/i18n/utils.ts). So under Node and SSR, UI strings degraded to raw translation keys with nothing logged.

The locale is now a TypeScript module, matching the existing messenger/src/i18n/en.ts precedent. The exported uiTranslations declaration is byte-identical to before.

Verification

• Imported every emitted module of all five packages under plain Node: 209 modules, 0 resolver errors.
• Declaration files carry full specifiers too, so consumers on node16/nodenext typecheck.
• lint, typecheck, test:coverage, build and publint all green; the new ui.ts is at 100% coverage and present in LCOV.
• Re-ran the repro above against the freshly published tarballs from the registry: both silent, and core's internal loadUITranslations() now returns "Main Menu" instead of {}.

Note on ordering

These versions are already on npm (messenger@0.1.1, ui@0.6.1, mdx@0.3.1), published from this branch. Please merge without squashing so the commit SHA referenced in the CHANGELOG entries (332db0b) stays valid.

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