Improve visual parity report evidence

[chubes4]

Summary

• extend the PHP transformer visual parity report contract with optional reason, bucket, pattern, confidence, property evidence, and capture diagnostics fields
• preserve editor-valid core/button custom font-size classes
• preserve class-owned grid/flex layout semantics in transformer output
• keep visual report schema/docs in sync with the additive contract fields

Testing

• composer exec -- php tests/contract/run.php

AI assistance

• AI assistance: Yes
• Tool(s): gpt-5.5 via OpenCode
• Used for: Implementing transformer contract/layout fixes, tests, and PR preparation.