Clean up canvas runtime preservation

[chubes4]

Summary

• omit non-runtime decorative/static canvas instead of preserving it as raw HTML
• preserve only explicit runtime canvas selectors as bounded runtime islands
• keep scripted canvas classified as runtime island preservation

Testing

• composer test in php-transformer
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode general subagents
• Used for: r15 matrix analysis, implementation, and tests