• perf: cache toResolveHierarchy results per (code, fallbackCode) pair. The hierarchy resolver runs on every t() call and calls Intl.getCanonicalLocales multiple times, which showed up prominently when profiling render-heavy UIs (e.g. virtualized data grids); with the cache the per-call cost drops from ~886 ns to ~41 ns. The cache is invalidated automatically when options.fallbackLng changes (reassignment or in-place array mutation); if you mutate other resolution-relevant options at runtime (load, lowerCaseLng, cleanCode, nonExplicitSupportedLngs), call i18next.services.languageUtils.clearCache() afterwards. Function-valued fallbackLng and per-call array/object fallbackLng options are never cached, so dynamic fallbacks keep working as before. Thanks @equaterina (#2444).
• chore: update all devDependencies (Babel stays on 7.x until @rollup/plugin-babel supports 8, eslint on 9.x for neostandard). Removed the unused coveralls package (CI uses the Coveralls GitHub Action) and replaced sinon with nise + vitest.spyOn in the v1 compatibility tests, which resolves all open npm audit findings (0 vulnerabilities) and should close the dependabot alerts on the lockfile.