Add CSS asset reference reporting

[chubes4]

Summary

• Add CSS @import and @font-face URL reference extraction.
• Surface reference metadata through artifact, compiled-site, and materialization-plan reports.
• Add parity/contract coverage for CSS import and font references.

Verification

• composer test:canonical
• composer test:parity
• composer test
• git diff --check

AI assistance

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