feat(phpunit): expose canonical discovery-only results

[chubes4]

Summary

• Add discovery-only to wordpress.phpunit and its recipe builder/CLI contract.
• Reuse WP Codebox's canonical PHPUnit XML discovery for stable, schema-bound file identities without WordPress, component, project bootstrap, or test execution.
• Keep discovery recipes mount-only, reject selectors and mixed workflows, and return wp-codebox/phpunit-discovery/v1 directly.
• Cover custom suffixes, explicit files, exclusions, empty/error boundaries, recipe setup isolation, and direct result transport.

Closes #2272.

Consumer: Extra-Chill/homeboy-extensions#2647.

Verification

• npm run build
• npm run test:phpunit-discovery-only
• npm run test:playground-phpunit-readonly-cache-integration
• npm run test:phpunit-structured-evidence
• npm exec -- tsx tests/phpunit-project-autoload.test.ts
• npm run test:phpunit-runtime-rejection
• git diff --check

AI assistance

• Model: GPT-5.6 Sol
• Tools: OpenCode, Homeboy, and WP Codebox
• Used for: Root-cause analysis, implementation, adversarial review, and verification. Chris Huber remains responsible for every line.

Also closes #2274 by replacing its scheduler-dependent 500ms assertion with a semantic rejection contract and a separate 5s deadlock guard.