Babel 8 support: @babel/register deadlocks with stdin program entry

[laat]

Problem

Bumping the babel-register / flow-type examples to Babel 8 hangs readme-assert forever (dependabot PRs #159 and #160 hung CI for the full 6-hour timeout; #161 reverted the bump).

readme-assert pipes each code unit to a child node process via stdin with an explicit --input-type flag (src/run.ts). With @babel/register 8 preloaded, that child deadlocks before executing anything.

Minimal repro

npm i @babel/core@8 @babel/register@8
echo "console.log('x')" | node --input-type=commonjs --require @babel/register -
# hangs forever; with Babel 7 it prints "x" and exits

What's known

• Affects @babel/register 8.0.0 and 8.0.1 (both tested in a clean project).
• The trigger is --input-type (either value) + program on stdin. Without --input-type, stdin entry works. -e and file entries work.
• The preload mechanism doesn't matter: --require and --import both deadlock identically.
• Babel 8 moved the register transform to a worker thread with blocking sync messaging ([babel 8] Move @babel/register transform to a separate worker babel/babel#14025). --input-type routes the stdin entry through the ESM loader, and that path deadlocks against the hook worker.
• Not yet reported upstream on babel/babel (searched issues + web, 2026-07).

Options

1. Report upstream to babel/babel and wait for a fix (examples stay on Babel 7; dependabot ignore rules for @babel/* majors are in place since Bump the all-dependencies group across 1 directory with 13 updates #161).
2. Switch readme-assert from stdin to temp-file execution: write each unit to a scratch file whose extension carries the type (.mjs/.cjs/.mts/.cts) instead of using --input-type. Sidesteps the bug entirely, but touches the [stdin] path rewriting and sourcemap handling in run.ts.

[laat]

Reported upstream: babel/babel#18130