Playground PHPUnit bootstrap fails when Composer platform check writes to undefined STDERR

## Problem

A Homeboy-scoped WordPress PHPUnit run using WP Codebox Playground fails before any tests execute when a staged plugin's Composer `platform_check.php` reports a platform mismatch. The generated check calls `fwrite(STDERR, ...)`, but `STDERR` is undefined in the Playground eval context.

## Reproduction

Run a WordPress PHPUnit recipe against a Composer-backed plugin whose staged dependency platform check fails. Current observed command:

```sh
homeboy review test extrachill-api --path /var/lib/datamachine/workspace/extrachill-api@feat-227-concert-privacy --placement local --skip-lint -- --filter 'Concert_Tracking_RoutesTest|User_Settings_Privacy_RoutesTest|UserLocalSceneAdaptersTest'
```

Bootstrap fails in `load_component`:

```text
Error: Undefined constant "STDERR" at /wordpress/wp-content/plugins/extrachill-api/vendor/composer/platform_check.php:17
```

The trace reaches the plugin Composer autoloader from the Playground internal evaluator, before PHPUnit can run.

## Expected

The test environment should expose CLI stream constants when executing CLI-style PHPUnit workloads, or capture/report the underlying Composer platform mismatch without replacing it with an undefined-constant fatal.

## Impact

No test assertions execute, and the actual dependency/platform diagnostic is obscured by the missing runtime constant.

Observed with WP Codebox 0.12.14, `@automattic/wp-codebox-core` 0.12.14, `@automattic/wp-codebox-playground` 0.12.14, WordPress Playground latest.

Reproduce the failure with the Homeboy review command and trace the Playground internal evaluator into load_component and the staged plugin's Composer platform_check.php. Check how CLI-style PHPUnit workloads expose stream constants and how the platform mismatch is reported; done means the underlying Composer diagnostic is preserved without an undefined-constant fatal before PHPUnit runs.

Written by the indexing model from the issue text.