Preserve generic runtime fallback metadata

[chubes4]

Summary

• preserve bounded script body metadata in runtime fallback diagnostics
• add fixture coverage for script body preservation and checkbox form control metadata
• keep this upstream primitive generic and free of downstream custom block behavior

Testing

• composer test
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Implementing generic fallback metadata coverage and running focused verification.