fix(parsing): case-insensitive arXiv idno match; fix tmp_path types; add extract_structure test

Review-Gate finding: GROBID emits type="arXiv" (camel-case), not "arxiv".
XPath literal match silently returned empty strings for all real responses.
Fix: iterate idno elements and compare .lower() == "arxiv".
Test fixture updated to reflect real GROBID output (type="arXiv").

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

codex/parsing/grobid.py

@@ -84,9 +84,12 @@ def extract_references(
         doi_el = bib.find(f".//{_TEI_NS}idno[@type='DOI']")
         doi = _text(doi_el)
 
-        # arXiv ID
-        arxiv_el = bib.find(f".//{_TEI_NS}idno[@type='arxiv']")
-        arxiv_id = _text(arxiv_el)
+        # arXiv ID — GROBID emits type="arXiv" (camel-case); match case-insensitively
+        arxiv_id = ""
+        for idno_el in bib.iter(f"{_TEI_NS}idno"):
+            if idno_el.get("type", "").lower() == "arxiv":
+                arxiv_id = (idno_el.text or "").strip()
+                break
 
         results.append(
             {