turbopack: mark injected chunk scripts as async for React DOM dedupe

[gurkerl83]

What?

Set script.async = true on JS chunk scripts created by Turbopack's DOM runtime backend.

Why?

Client-side App Router navigations request the same route chunk twice when Turbopack and React DOM both try to load it.

The reproducer and investigation are here:

• Reproducer: https://github.com/gurkerl83/next-app-router-duplicate-chunk-nav-repro
• Repro README: https://github.com/gurkerl83/next-app-router-duplicate-chunk-nav-repro/blob/main/README.md
• Investigation notes: https://github.com/gurkerl83/next-app-router-duplicate-chunk-nav-repro/blob/main/RESOLUTION.md

In particular, the investigation notes describe
"Option A: Add async Attribute to Turbopack's Script Tags", which is what this PR implements.

React DOM reuses hoistable scripts by src and looks them up in the DOM via script[async][src="..."]. Turbopack's DOM chunk loader inserted matching chunk scripts without explicitly setting async, so React DOM misses an already-present script and appends a second one for the same URL.

How?

Set async explicitly on Turbopack-inserted chunk scripts so they participate in the same DOM dedupe path that React DOM uses for hoistable scripts.

Dynamically inserted scripts already load asynchronously, so this only makes the DOM representation explicit without changing load behavior.

This change is in Turbopack's DOM runtime backend, which is used for both development and production DOM chunk loading.

Verification

Validating this change against the reproducer requires:

1. In the local next.js checkout, run a full rebuild so the Turbopack native
   binary is rebuilt with the updated embedded DOM runtime sources:

pnpm build-all

2. In the reproducer app, build it against that local native binding:

NEXT_TEST_NATIVE_DIR=<path-to-local-nextjs-checkout>/packages/next-swc/native pnpm next build

Without NEXT_TEST_NATIVE_DIR, the reproducer may resolve a different native binding and not include the patched runtime.

Fixes #92689

[nextjs-bot]

Allow CI Workflow Run

• approve CI run for commit: ce0072e

Note: this should only be enabled once the PR is ready to go and can only be enabled by a maintainer

[codspeed-hq]

Merging this PR will not alter performance

✅ 17 untouched benchmarks
⏩ 3 skipped benchmarks¹

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_{Comparing gurkerl83:fix-turbopack-duplicate-chunk-script-load (ce0072e) with canary (ef3f853)}

Footnotes

1. 3 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩