Resolve local image asset references

[chubes4]

Summary

• Pass source-relative image asset metadata from artifact compilation into the HTML transformer.
• Resolve known local image src and srcset references to materializable manifest paths while preserving missing/remote candidates.
• Add responsive/nested image fixture coverage and update materialization expectations for asset rewrite candidates.

Testing

• cd php-transformer && composer test
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Drafted implementation and tests for generic Blocks Engine image asset parity; Chris reviews and owns the change.