Harden php-transformer plugin helper contract

[chubes4]

Summary

• Harden the php-transformer plugin bootstrap contract so WordPress-facing helpers are checked against the equivalent canonical class APIs.
• Normalize only per-call timing metrics before comparing result envelopes.
• Keep the release-readiness cleanup limited to product-neutral test coverage; no release, tag, or publish actions.

Verification

• composer validate from php-transformer/
• composer test from php-transformer/
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Inspecting release-readiness surfaces, drafting the contract-test hardening, running verification, and preparing this PR.