fix: use process.stdout.write for timing markers to bypass Vitest interception

[Saga4]

Summary

• Changes capturePerf timing markers from console.log to process.stdout.write
• Ensures timing markers are always captured by the parent process

Problem

Vitest intercepts console.log output and may not pass it to the subprocess stdout. This caused timing markers in capturePerf to not be captured by the parent process, resulting in "overall summed benchmark runtime is 0" errors.

Solution

Changed from:

console.log(`!######${testStdoutTag}:${durationNs}######!`);

To:

process.stdout.write(`!######${testStdoutTag}:${durationNs}######!\n`);

process.stdout.write writes directly to the stdout stream, bypassing any console interceptors.

Test plan

• Added test file runtime/__tests__/stdout-write.test.js
• Test verifies timing markers use process.stdout.write
• Test verifies markers are NOT using console.log
• Manual testing with Vitest-based projects

Generated with Claude Code

[Saga4]

Closing - needs more verification before merging