Normalize HTML fallback diagnostics

[chubes4]

Summary

• Add reusable fallback diagnostic builder for HTML transforms.
• Normalize key fallback paths while preserving existing fields.
• Project severity/runtime/recoverability/actionability/materialization hints in conversion reports.

Verification

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

AI assistance

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