Harden PHPUnit suite and mount contracts

[chubes4]

Summary

• parse PHPUnit runner options and --testsuite selectors in one generated-PHP pass
• make explicit suite selection authoritative over the default discovery root
• prove file-only suites discover exactly their configured files
• extend the real Playground integration to cover readonly parent plus readonly nested overlay visibility and host isolation

Fixes #2158. Follow-up hardening for #2153 and #2154.

Verification

• npm run build
• npm exec -- tsx tests/phpunit-project-autoload.test.ts
• npm run test:phpunit-structured-evidence
• npm run test:phpunit-runtime-rejection
• npm exec -- tsx tests/playground-direct-mount-materialization.test.ts
• npm exec -- tsx tests/staged-input-materialization.test.ts
• npm exec -- tsx tests/mount-materialization.test.ts
• npm exec -- tsx tests/recipe-runtime-setup-staged-materialization.test.ts
• npm exec -- tsx tests/playground-readonly-mounts.test.ts
• npm exec -- tsx tests/playground-readonly-mounts-integration.test.ts

Full WPCOM TeamCity parity at comparator SHA 7ae9ae21ca30f7fc9724aecfb54a16f674f82a23:

• 677/677 suites passed
• 674/674 WP Codebox suites passed
• 3/3 native suites passed
• zero failures and zero timeouts
• 4 shards x 6 workers, timeout multiplier 3

Reproduction: run node scripts/run-wpcom-teamcity-parity.mjs --mode execute --allow-execute true --manifest artifacts/phpunit-profiles/teamcity-parity-manifest-7ae9ae21-corrected.json --concurrency 6 --shards 4 --timeout-multiplier 3 from the WPCOM Codebox parity workspace with the certified runtime inputs.

AI Assistance

OpenAI gpt-5.6-sol via OpenCode identified the file-only suite edge case, consolidated generated-PHP option parsing, expanded executable integration coverage, and ran local and full WPCOM verification. Chris Huber remains responsible for the change.