Adopt next/root-params in Next.js RSC example and tutorial

Next.js 16.3 introduced next/root-params, which lets any Server Component read root dynamic segments (e.g. [lang]) without prop drilling. This simplifies Lingui's RSC setup - no core changes needed, the improvements are in the example and docs.

Current state

• Every page/layout in examples/nextjs-swc takes params, awaits it, and passes lang to initLingui(lang).
• The RSC tutorial documents this caveat: "you need to repeat the setI18n in every page and layout… Please let us know if there's a known better way."

Proposed changes

Example (examples/nextjs-swc):

• Upgrade Next.js 15.0.1 → 16.3+
• Make initLingui() read the locale via next/root-params internally, so pages/layouts/generateMetadata call it with zero args and stop threading params

Docs (tutorials/react-rsc):

• Update snippets to the root-params pattern; keep the params-prop approach for Next.js < 16.3
• Rewrite the "Pages, Layouts and Lingui" section (the per-page call remains due to React cache ordering, but becomes a zero-arg one-liner)
• Document caveats: not available in Client Components, Server Actions, or Route Handlers - pass the locale explicitly there
• Optionally: note that root-param getters are tracked in 'use cache' cache keys, so getI18nInstance(await lang()) inside cached functions is keyed per locale

Notes

Lingui's server-side useLingui()/Trans are sync and can't lazily resolve the locale themselves, so initLingui() per page/layout is still required. Removing it entirely would need an async init path in @lingui/react/server - out of scope here.

Start in examples/nextjs-swc by inspecting the current Next.js version, initLingui usage, and params flow, then review tutorials/react-rsc for the existing RSC guidance. Done means the example uses next/root-params with the documented version boundary, and the tutorial covers zero-argument initialization plus the listed Client Component, Server Action, and Route Handler caveats.