docs(paper): refresh JOSS paper for current library and submission requirements

[igerber]

Summary

• Refresh paper.md / paper.bib for the JOSS submission so they reflect the current library and meet current JOSS requirements.
• Correct estimator count 17 -> 19; enumerate all 19 estimators by name and list diagnostics (Goodman-Bacon, Honest DiD, placebo tests, pre-trends power) separately.
• Add a mandatory AI Usage Disclosure section (tool/version, scope of assistance, human-verification affirmation) per current JOSS policy.
• Cite the companion arXiv preprint Gerber (2026, 2605.04124) in the Summary, Statement of Need, and Survey-weighted inference section.
• Soften an overstated survey-validation tolerance (<1e-10 -> test gaps <1e-8, typically <1e-10) and correct "three federal datasets" (the API dataset is California state data, not federal).
• Add 4 bib entries (Gerber2026, Abadie2010, Butts2021, deChaisemartin2026) and add arXiv/Springer DOIs across the bibliography.
• Remove the Acknowledgments section (Wenli Xu remains credited in CONTRIBUTORS.md); update date to 3 July 2026.

Methodology references (required if estimator / math changes)

• Method name(s): N/A - no methodology or code changes (paper files only)
• Paper / source link(s): N/A
• Any intentional deviations from the source (and why): None

Validation

• Tests added/updated: No test changes
• Backtest / simulation / notebook evidence (if applicable): N/A. Verified all 24 paper citations resolve to paper.bib (no orphan/duplicate keys); paper claims cross-checked against docs/benchmarks.rst and tests/test_survey_real_data.py.

Security / privacy

• Confirm no secrets/PII in this PR: Yes

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Generated with Claude Code

[github-actions]

Overall Assessment

✅ Looks good — no unmitigated P0/P1 findings. This PR is documentation/bibliography only; I found no estimator, math, weighting, variance/SE, assumption, or default-behavior change.

Executive Summary

• The 19-estimator count is consistent with the current README estimator inventory.
• Citation keys in paper.md all resolve in paper.bib; no orphan bib entries detected.
• Survey-validation wording is more accurate than the older benchmark prose and matches the 1e-8 test tolerance.
• Two minor documentation issues remain: the AI disclosure overstates R-validation coverage, and the paper date is future-dated relative to today, June 27, 2026.

Methodology

• Severity: N/A. Impact: No methodology implementation changed. The affected files are paper.md and paper.bib only, and the estimator inventory aligns with the documented 19 estimators in README.md:L101-L119. Concrete fix: None.

Code Quality

• Severity: N/A. Impact: No code changed. Concrete fix: None.

Performance

• Severity: N/A. Impact: No runtime behavior changed. Concrete fix: None.

Maintainability

• Severity: P3. Impact: The AI disclosure says numerical results were verified against R packages “for every estimator with an R equivalent” (paper.md:L111-L113), but project tracking still lists external-reference/parity gaps for StaggeredTripleDifference, WooldridgeDiD, SyntheticDiD bootstrap variance, and HAD pretests (TODO.md:L109-L122). These limitations are tracked, so this is informational, not a blocker. Concrete fix: soften to “where reference fixtures exist” or name the exceptions.

Tech Debt

• Severity: P3. Impact: The above validation gaps are already tracked under TODO’s external-reference section, so this PR does not introduce untracked methodology debt. Concrete fix: None required for approval.

Security

• Severity: N/A. Impact: No secrets or security-sensitive changes found in the changed paper/bib content. Concrete fix: None.

Documentation/Tests

• Severity: P3. Impact: paper.md:L17 sets date: 3 July 2026, which is future-dated relative to the review date, June 27, 2026. If submitted before July 3, the manuscript metadata will be inaccurate. Concrete fix: set the date to the actual submission/publication date, or defer the date update until then.

• Severity: P3. Impact: No tests are necessary for this docs-only change; local citation-key checks passed. Concrete fix: None.