Extract artifact path helper

[chubes4]

Summary

• Add reusable ArtifactPath helper for artifact-local paths.
• Replace duplicated path/reference resolution in compiler and normalizer paths.
• Preserve existing behavior with focused contract coverage.

Verification

• composer test:canonical
• composer test
• php -l src/Path/ArtifactPath.php
• 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.