Add runtime island reporting

[chubes4]

Summary

• report preserved native DOM runtime islands as first-class transformer metadata
• include selectors, source snippets, runtime requirements, required scripts, and preservation reasons
• project runtime island metadata through conversion reports for downstream 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