Provision multisite PHPUnit runtimes

[chubes4]

Summary

• provision multisite: true PHPUnit recipes with Playground's native enableMultisite blueprint step
• provide a no-port http://localhost site URL required by Playground multisite, while preserving explicit preview overrides
• expose WordPress's $wp_rewrite global to the function-scoped test installer include
• run the existing real PHPUnit integration in multisite mode and assert is_multisite()

Why

Passing multisite=1 only to wordpress.phpunit was insufficient because Playground had already booted WordPress as single-site. Native multisite provisioning also requires a canonical site URL without the dynamic CLI port. Once provisioned, WordPress's test installer needs $wp_rewrite imported into the function scope where WP Codebox includes it.

Closes #1898.

Verification

• npm run build
• npx tsx tests/phpunit-project-autoload.test.ts
• npx tsx tests/playground-phpunit-readonly-cache.integration.test.ts
• Extra Chill Users network-only suite boots and runs: 257 tests discovered, 149 passed, 7 skipped; remaining failures are missing Extra Chill dependency fixtures rather than multisite/runtime failures