docs(paper.bib): add 4 verified DOIs flagged by JOSS editorialbot

JOSS editorialbot's `check references` run on issue #10604 flagged
9 entries as "MISSING DOIs" (suggestions only — needed §10
verification). Triage results:

ADD (4 entries, §10 dual-source verified — Crossref metadata match +
doi.org redirect to canonical publisher URL):

- `fiorini2024simple` += doi 10.2139/ssrn.4772584
   (CESifo WP 11011 also deposited on SSRN; same authors / year /
    title; SSRN URL pattern 4772584 confirmed via doi.org redirect)
- `santanna2020doubly` += doi 10.1016/j.jeconom.2020.06.003
   (JoE 219(1):101-122 published version; Crossref returns exact
    journal / volume / issue / pages / authors / title / 2020 match.
    Note: editorialbot suggested SSRN WP DOI 10.2139/ssrn.3293315
    but the canonical reference is the JoE published article, so
    we add the JoE DOI instead per §10 honesty)
- `mantel1959statistical` += doi 10.1093/jnci/22.4.719
   (classic 1959 Mantel-Haenszel paper; doi.org → academic.oup.com
    landing page URL contains volume=22, issue=4, page=719 — exact
    bib match. Crossref returns empty author list, common quirk
    for pre-1990 OUP papers, but JNCI metadata is unambiguous)
- `abadie2025harvesting` += doi 10.3386/w34550
   (NBER WP 34550 DOI; Crossref returns all 4 authors and exact
    title match, doi.org → nber.org PDF for w34550)

SKIP with documented reason (3 entries — editorialbot's suggestion
points to a different document version than what the bib cites):

- `abadievives2022synthetic`: bib cites 2022 arXiv:2203.06279; the
  suggested 10.1017/9781009589727.010 is the 2025 Cambridge book-
  chapter published version. Different documents — would silently
  upgrade the citation. Leave to author decision.
- `westfall1993resampling`: bib cites the 1993 Wiley book (ISBN
  978-0-471-55761-6). The suggested 10.2307/2533464 is a 1994 JSTOR
  entry — appears to be a book review, not the book itself.
- `webb2014reworking`: bib cites the 2014 QED Working Paper #1315;
  the suggested 10.1111/caje.12661 is the 2023 CJE published version.
  Different documents.

False positives in editorialbot's list (NOT in CHANGELOG above):

- `ado2019shift` already has doi 10.1093/qje/qjz025 (the QJE 2019
  published version). editorialbot's suggestion 10.3386/w24944 is
  the NBER WP precursor — published version is canonical, no change.
- `schwab2013ltmle` already has the suggested DOI
  10.32614/cran.package.ltmle — editorialbot's title-search round-
  tripped its own result, no change.

Verification trail:
- Crossref API: 4/4 returned matching authors / year / title
- doi.org HEAD/GET: all 4 DOIs resolve to canonical publisher URLs
  (SSRN/Oxford UP return 403 to non-browser HEAD but the redirect
   chain itself confirms registration; Elsevier + NBER return 200)
- Local audit gates 0/1/2 pass; gate 3 DOI subset 75/75 resolved
  (gate 3 arXiv subset hit local rate-limit during this session,
   unrelated to these changes — GitHub Actions runner will re-verify
   with a fresh IP; CI on the v1.15.6 commit already showed 0
   mismatch / 0 unresolved before this commit)
- pybtex strict parse: 551 entries, 0 empty keys
- pandoc citeproc: exit 0
- No bibtex duplicates / dangling refs / coverage regressions

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: brycewang-stanford

paper.bib

@@ -1201,7 +1201,8 @@ @techreport{fiorini2024simple
   type={CESifo Working Paper},
   number={11011},
   year={2024},
-  url={https://hdl.handle.net/10419/296100}
+  url={https://hdl.handle.net/10419/296100},
+  doi={10.2139/ssrn.4772584}
 }
 
 @misc{serenini2024spatial,
@@ -3211,7 +3212,8 @@ @article{santanna2020doubly
   year={2020},
   publisher={Elsevier},
   eprint={1812.01723},
-  archivePrefix={arXiv}
+  archivePrefix={arXiv},
+  doi={10.1016/j.jeconom.2020.06.003}
 }
 
 @article{cohort_anchored2025,
@@ -4062,7 +4064,8 @@ @article{mantel1959statistical
   volume={22},
   number={4},
   pages={719--748},
-  year={1959}
+  year={1959},
+  doi={10.1093/jnci/22.4.719}
 }
 
 @misc{schroder2024conformal,
@@ -4358,7 +4361,8 @@ @techreport{abadie2025harvesting
   institution={National Bureau of Economic Research},
   type={NBER Working Paper},
   number={34550},
-  year={2025}
+  year={2025},
+  doi={10.3386/w34550}
 }
 
 @article{correia2020fast,