Sharpen raw HTML canvas fallback diagnostics

[chubes4]

Summary

• Treat raw HTML as a canvas-specific runtime fallback instead of a generic unsupported element.
• Preserve bounded sanitized canvas markup and safe id/class/size attributes in fallback metadata for runtime mapping.
• Surface script dependency hints when scripts may target the canvas element, while continuing to avoid core/html/raw canvas block output.

Testing

• php tests/contract/run.php
• composer test

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Investigation, implementation, tests, and PR preparation.