Classify php-transformer conversion preservation

[chubes4]

Summary

• add backwards-compatible conversion classification metadata to php-transformer diagnostics and reports
• distinguish native block conversion, editable approximation, runtime island preservation, and unsupported loss
• expose classification summaries for downstream SSI/matrix consumers

Testing

• composer test

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode general subagents
• Used for: architecture refactor drafting, implementation, conflict merge, and tests