[v4] Integrate Crossref retraction API for structured retraction checking

[cpitzi]

Summary

Replace web-search-based retraction checking with direct Crossref REST API queries for structured retraction metadata (update-to, update-type fields).

See roadmap/v4-features.md (Priority 4) for full specification.

Current State

The v3 prompt checks for retractions via web search against Retraction Watch. This works but is indirect — it depends on search result quality and Retraction Watch coverage.

Target State

For every DOI-bearing reference, query the Crossref API for:

• update-to relationships (retraction notices, corrections, expressions of concern)
• update-type field values
• Retraction notice DOI and date

Implementation Approach

This is most naturally implemented as part of the pipeline decomposition (Stage 2: Sonnet + programmatic verification). In the current monolithic prompt, it can be approximated by instructing Claude to query specific Crossref API URL patterns via web fetch.

Acceptance Criteria

• Every DOI-bearing reference checked for Crossref retraction/correction metadata
• Retraction notice DOI and date included in forensic notes
• Distinguishes retraction, correction, and expression of concern
• Handles partial retractions and editor's notes
• Works in both monolithic prompt and pipeline architecture

Dependencies

• Crossref REST API (free, polite pool — no authentication required for basic queries)
• Pipeline architecture (ideal but not required)