[fix]: refresh ARM64 emulation before sandbox releases

[tlgimenes]

What is this contribution about?

The multi-architecture sandbox release failed when Node 26.10.0 started npm under the runner's preinstalled ARM64 emulator: QEMU raised an illegal-instruction signal and exited 132. Register fresh ARM64 emulation before Buildx and reset the existing handler. Add a read-only PR check that builds the Dockerfile's Node stage and exercises Node and npm under ARM64 emulation.

How did you verify your code works?

The new Ubuntu ARM64 Node/npm smoke check passes. All PR checks are green. Using the CI-pinned Bun 1.3.14, bun run verify --parallel=2 passes formatting, lint, unused-code checks and all 10,295 tests across 996 files. The default-parallel runs hit timing assertions in unchanged tests; the final run uses the same full suite and assertions with a two-worker resource cap.

Both YAML files parse and all embedded shell blocks pass syntax checks. The installed actionlint input metadata is stale for the new emulator action; its reset input was verified against the exact upstream v4 schema. The patch changes only the release workflow and its smoke check; it does not change the image's Node version, runtime code, signing or deployment conditions.

How to Test

1. Run the new Node and npm under ARM64 emulation CI check on an Ubuntu AMD64 runner.
2. Verify that the Dockerfile's Node stage loads as ARM64 and both node --version and npm --version complete.
3. After merging, dispatch the existing sandbox publication workflow at the tested main revision and verify that both image variants publish before the deployment notification.

Migration Notes

This repairs the failed publication of the current sandbox version. No image version bump is included. A workflow-only merge does not trigger the existing release path filter, so publication must be dispatched after checks pass.

Review Checklist

• PR title is clear and descriptive
• Local verification and ARM64 smoke CI pass
• Existing signing and deployment conditions are preserved