Route groups / segments with non-Latin-1 characters crash with `InvalidCharacterError: Invalid character` (regression from Next 15)

Link to the code that reproduces this issue

https://github.com/hyonun321/nextjs-non-latin1-route-group-repro

To Reproduce

1. npx create-next-app@latest repro --app --ts --no-tailwind --no-eslint

2. Create a route group with a non-ASCII name and a page inside it:

   app/(안녕)/hello/page.tsx
   

   export default function Page() {
     return <h1>hello</h1>
   }
   

3. next dev and open /hello → 500, terminal prints
   Error [InvalidCharacterError]: Invalid character

4. next build → prerendering fails:

   Error [InvalidCharacterError]: Invalid character
   Error occurred prerendering page "/hello".
   Export encountered an error on /(안녕)/hello/page: /hello, exiting the build.
   

Same result with --webpack, so it is not bundler specific.

Current vs. Expected behavior

Expected: same as Next 15 — a route group name is never part of the URL, so
its character set should not affect routing or rendering.

Also affects Japanese (日本), Cyrillic (тест), Latin-1 supplement (café)
and emoji (😀) — anything outside the Latin-1 range.

Root cause

encodeToFilesystemAndURLSafeString in
packages/next/src/shared/lib/segment-cache/segment-value-encoding.ts calls
btoa(value) on the raw segment name. btoa only accepts Latin-1 input and
throws a DOMException (InvalidCharacterError) otherwise. The segment cache
is now on by default, so every non-ASCII segment name hits this path.

The repo already has a UTF-8-safe variant of the same base64url encoding in
shared/lib/router/utils/cache-busting-search-param.ts (TextEncoder → binary
string → btoa).

Provide environment information

Operating System: Windows 11 Pro 10.0.26200
Binaries: Node 22.14.0
Relevant Packages: next 16.3.3, react 19.x

Which area(s) are affected? (Select all that apply)

Route Groups, Dynamic Routes

Which stage(s) are affected? (Select all that apply)

next dev (local), next build (local), next start (local)

Additional context

Note this is separate from #10084 (non-ASCII URL segments such as
app/소개/page.tsx → /소개 returning 404), which reproduces on Next 15 as
well and is a routing/matching problem, not an encoding crash. This issue is
only about the Next 16 regression for non-ASCII route group / segment names
used in the segment cache key.

Fix PR: https://github.com/vercel/next.js/pull/97884

Previously filed as #97883 (auto-closed: body headings did not match the issue form) and #97886 (missing area labels). This one supersedes both; same content.

Start with packages/next/src/shared/lib/segment-cache/segment-value-encoding.ts and the UTF-8-safe encoding variant in shared/lib/router/utils/cache-busting-search-param.ts. Run the linked reproduction with a non-ASCII route group using next dev and next build. Done means the route renders and prerenders successfully, matching the Next 15 behavior.