Add TESTING.md for local, CI, and release-branch validation rules

Summary

The repo does not have a dedicated TESTING.md that explains how Perform validation should be run and reported across local, CI, and release-branch work.

Evidence

• Repo-root audit found no TESTING.md.
• Validation commands exist in README.md, but there is no durable testing/proof guide for product PO and worker behavior.
• Current blocker work already depends on distinguishing local CLI proof, CI proof, and blocked browser screenshot paths.

Scope

• Add TESTING.md with the canonical validation matrix for PHP, JS/CSS, build, packaging, and browser checks.
• Cover local contributor workflows, CI expectations, and release-branch validation.
• Explain when wp-env, Studio, studio wp, and GitHub Actions artifacts are the right proof source.
• Require explicit documentation of any unrun validation step or blocked proof path.

Acceptance criteria

• One canonical testing doc exists in the repo root.
• Validation commands in the doc match current repo tooling.
• The doc explains proof-source hierarchy for local, CI, and screenshot/browser checks.
• Proof gaps and blocked environments must be recorded explicitly instead of implied.