Fix federated article search offset/sort semantics and add correctness spec

[imaurer]

Summary

• Apply offset after merging and deduplicating both federated legs (PubTator3 + Europe PMC), not per-leg, so paginated windows are stable and correct
• Sort merged results by the caller's requested sort order (relevance, citations, date) before applying offset and truncation
• Propagate the same sort+offset logic through surviving-leg fallback paths so single-source results are consistent with the two-source path
• Guard against deep offsets that exceed the fetch capacity bound: offset + limit > 1250 returns an explicit InvalidArgument error rather than silently returning an incorrect window
• Normalize partial dates (YYYY, YYYY-MM) to full ISO-8601 strings (YYYY-01-01, YYYY-MM-01) so date comparisons are lexicographically correct across mixed-precision values
• Extend spec/06-article.md with a deep-offset rejection contract test

Test plan

• cargo test --lib entities::article — 31 article unit tests pass, including new tests for offset-after-merge, sort ordering, partial date normalization, retraction filtering, and surviving-leg sort-before-offset
• cargo test — 584 tests pass, 0 failed
• cargo clippy -- -D warnings — clean
• cargo fmt --check — clean
• make spec — 98 passed, 5 skipped (spec/06-article.md deep-offset guard test included)
• Manual smoke: biomcp search article -k melanoma --limit 50 --offset 1201 fails with expected error; --offset 1200 succeeds