Fix CLI streams in Playground PHP runtimes

[chubes4]

Summary

• define PHP's standard CLI stream constants at the shared Playground command bootstrap before WordPress and test dependencies load
• reuse the same stream primitive in the dedicated WP-CLI script instead of maintaining a second copy
• cover the cli namespace fallback contract in the real Playground PHPUnit integration fixture

Root cause

PHP-WASM does not define the CLI-only global STDIN, STDOUT, or STDERR constants. WP Codebox already polyfilled them for the dedicated wordpress.wp-cli command, but wordpress.phpunit enters through the shared Playground PHP bootstrap instead. Tests that load WP-CLI formatter libraries therefore reached php-cli-tools without global STDOUT.

PHP reports the failed unqualified lookup from namespace cli as the exact fatal:

Error: Undefined constant "cli\\STDOUT"

A Playground probe confirmed both defined('STDOUT') and defined('cli\\STDOUT') were false before this change. The fix defines the global CLI primitive; PHP's normal namespace fallback then resolves unqualified STDOUT in cli code without defining a separate namespaced constant.

Layer ownership

This belongs to WP Codebox's Playground/PHP runtime bootstrap. It does not patch vendored WP-CLI code and does not add a consumer workaround in data-machine-events.

Verification

• npm run build
• npm run test:runtime-php-snippets
• npm exec -- tsx tests/phpunit-project-autoload.test.ts
• npm run test:playground-phpunit-readonly-cache-integration
• npm run test:production-boundary-enforcement
• git diff --check
• live wordpress.run-php Playground probe: global STDOUT/STDERR are defined resources and unqualified STDOUT resolves inside namespace cli

The broader runtime smoke group also exposed an unrelated pre-existing 25 ms timing assertion failure, tracked separately in #1904.

Closes #1901