Extract reference analyzer

[chubes4]

Summary

• Extract asset/reference analysis into a reusable ReferenceAnalyzer primitive.
• Preserve ArtifactCompiler behavior while reducing embedded reference parsing logic.
• Add contract coverage for HTML and CSS reference analysis.

Verification

• composer test

AI assistance

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