Report static interaction candidates

[chubes4]

Summary

• Add static interaction candidate detection to HtmlTransformer reports.
• Project candidates into conversion_report for consumers.
• Cover forms, details, controls, ARIA, tabs, accordion, modal, and carousel-like signals.

Verification

• composer test:canonical
• composer test:parity
• composer validate
• composer test

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Drafting and implementing the code/test changes.