[v4] Implement temporal impossibility checks

[cpitzi]

Summary

Flag citations where the stated publication year is impossible given the journal's existence or the manuscript's submission timeline.

See roadmap/v4-features.md (Priority 2) for full specification.

Detection Targets

• Reference dated before the journal's first issue (e.g., citing a 2019 article from a journal founded in 2022)
• Reference dated after the manuscript's submission date (unless verifiable as ahead-of-print/preprint)
• Volume/issue numbers that don't exist for the stated year (e.g., Volume 58 of a journal currently on Volume 53)

Implementation Approach

1. During Stage 2 verification, check journal founding date via Crossref journal metadata or NLM Catalog
2. Compare reference year against journal founding year
3. Compare reference year against manuscript submission date (if provided)
4. Check volume/issue plausibility against journal's publication history

Acceptance Criteria

• Flags references predating journal existence
• Flags future-dated references (with preprint/ahead-of-print exception)
• Flags impossible volume/issue combinations
• Does not false-flag journals with name changes (old name + old date is valid)
• Classified as High risk when combined with other anomalies, Elevated risk in isolation

Test Cases Needed

• Add 2-3 temporally impossible references to the adversarial test set or a dedicated test file