Add generic asset materialization rows

[chubes4]

Summary

• Add product-neutral write fields to php-transformer materialization plan asset rows.
• Preserve text/base64 payloads, target paths, media types, and hashes from normalized artifact assets.
• Cover website/theme artifact asset rows in contract and parity fixtures, and document the result-envelope contract.

Testing

• composer validate
• composer test
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Implementing php-transformer asset materialization plan rows, updating tests/docs, and running local verification.