fix(test): publish process markers atomically

[chubes4]

Summary

• publish the native interruption pause marker through a same-directory atomic rename
• publish the archive-cache child readiness marker through the same complete-payload contract
• prevent existence-based readers from observing empty or partial readiness payloads under concurrent smoke load

Evidence

• PR fix(browser): expect external editor presentation identities #2453 smoke run https://github.com/Automattic/wp-codebox/actions/runs/33530273032 failed with an empty JSON interruption marker
• v0.26.6 Homeboy release preflight failed independently with an empty archive-cache reference marker and rolled back before mutation

Verification

• npx tsx tests/playground-custom-archive-cache-process.test.ts
• node tests/execute-native-agent-task-interruption.test.mjs
• npm run build
• npm run check (342 commands, concurrency 8)

Fixes #2454.

AI assistance

OpenAI GPT-5.6 Sol via OpenCode correlated the two release-blocking failures, identified the shared readiness-publication race, implemented the atomic marker protocol, and ran focused and full concurrent verification. Chris Huber directed and owns the work.