feat(crossref): reference extraction as third citation fallback (R-B)

Add crossref.fetch_references(doi), which parses message.reference[].DOI into Citations (DOI-less book/unstructured refs are skipped — they cannot join the graph). Wire it as the third leg of the ingest citation fallback chain: OpenAlex-empty -> S2 -> Crossref, for DOI papers only (Crossref's works endpoint is DOI-keyed). Cited DOIs are normalized to the canonical bare lower-case form via _norm_cited_id.

As a fallback it fires only when OpenAlex and S2 both return no references, so it adds a forward-looking third source without changing the already-covered corpus.

Tests: fetch_references unit tests (DOI edges / unstructured skipped / no refs / 404) and ingest tests for both fallback directions. Full suite 361 passed; ruff + mypy clean. Live-validated: 33/40/24 refs for Springer/ACM/Wiley DOIs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

codex/sources/crossref.py

@@ -2,10 +2,11 @@
 
 Provides:
 - fetch_abstract: retrieve a work's abstract (JATS-stripped) by DOI.
+- fetch_references: retrieve a work's reference list (DOI edges) by DOI.
 
 Crossref is a *third* metadata source after OpenAlex and Semantic Scholar
-(DQ-2 / roadmap R-B). Requests use the Polite Pool (mailto query parameter)
-and are retried on 429/5xx with exponential back-off via tenacity.
+(DQ-2 abstracts / roadmap R-B references). Requests use the Polite Pool (mailto
+query parameter) and are retried on 429/5xx with exponential back-off via tenacity.
 """
 
 from __future__ import annotations
@@ -18,6 +19,7 @@ import httpx
 from tenacity import retry, retry_if_exception, stop_after_attempt, wait_exponential
 
 from codex.config import get_settings
+from codex.models import Citation
 
 logger = logging.getLogger(__name__)
 
@@ -89,3 +91,36 @@ def fetch_abstract(doi: str) -> str | None:
         raise
     message: dict[str, Any] = response.json().get("message", {})
     return _strip_jats(message.get("abstract"))
+
+
+def fetch_references(doi: str) -> list[Citation]:
+    """Fetch a work's reference list from Crossref by DOI (roadmap R-B).
+
+    Crossref carries publisher-deposited reference lists in ``message.reference``.
+    Each entry that cites a DOI-registered work exposes a bare ``DOI`` field;
+    only those become citation-graph edges (book / older references carry just
+    ``unstructured`` text with no resolvable id and are skipped). ``citing_id``
+    is the queried *doi* — the caller rewrites it to the canonical ``papers.id``
+    and normalises the cited DOIs (mirrors the Semantic Scholar reference path).
+
+    Parameters
+    ----------
+    doi:
+        A bare DOI (``"10.1007/s00454-019-00132-8"``).
+
+    Returns
+    -------
+    list[Citation]
+        One Citation per reference that carries a DOI. Empty on 404 or when no
+        references are deposited for the work.
+    """
+    url = f"{_BASE}/works/{doi}"
+    try:
+        response = _get(url)
+    except httpx.HTTPStatusError as exc:
+        if exc.response.status_code == 404:
+            return []
+        raise
+    message: dict[str, Any] = response.json().get("message", {})
+    references: list[dict[str, Any]] = message.get("reference") or []
+    return [Citation(citing_id=doi, cited_id=ref["DOI"]) for ref in references if ref.get("DOI")]