Retain non-fatal PHPUnit bootstrap exits

[chubes4]

Summary

• treat wp-load.php as an explicit completion boundary for PHPUnit commands
• buffer and persist bounded output when bootstrap terminates via die() or exit() without a PHP fatal
• preserve existing output after successful bootstrap
• extend the real Playground integration with an exit(1) MU-plugin fixture

Root cause

#1918 recorded only fatal error_get_last() types. Intelligence CI terminates before wp-load.php returns without a fatal payload, so the recorder returned without writing and downstream diagnostics remained empty even on current WP Codebox main.

Verification

• npm run build
• npm run test:phpunit-runtime-failure-diagnostics
• npm run test:playground-phpunit-bootstrap-failure-integration
• git diff --check

Fixes #1916