php-transformer: preserve class-owned vertical flex

[chubes4]

Summary

• keep class-owned vertical flex CSS under the preserved source class instead of synthesizing WordPress vertical flex layout attrs
• preserve explicit inline vertical flex layout mapping and existing horizontal flex/grid inference
• add a contract regression for class-owned vertical flex containers

Testing

• php php-transformer/tests/contract/run.php
• php php-transformer/tests/parity/run.php
• php php-transformer/tests/unit/corpus-detectors.php

AI assistance

• AI assistance: Yes
• Tool(s): gpt-5.5 via OpenCode
• Used for: Diagnosing the layout parity regression, implementing the transformer/test change, and running targeted verification.