Add Zenodo DOI badge, citation metadata, and identifiers

Zenodo minted DOIs for diff-diff when v3.1.3 was released:
- concept DOI: 10.5281/zenodo.19646175 (always-latest)
- v3.1.3 DOI: 10.5281/zenodo.19646176

Wires the concept DOI into the README badge row and the BibTeX citation
block; adds concept + versioned DOIs to CITATION.cff via `doi:` (top-level,
for tools like GitHub's "Cite this repository" button) and `identifiers:`
(for explicit concept-vs-versioned distinction). Also updates the BibTeX
author from "diff-diff contributors" to "Gerber, Isaac" to match
CITATION.cff's canonical authorship.

Docs audit (paper.md, docs/, llms guides) found no stale "no DOI" or
"pending" placeholder language to clean up.

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

Author: igerber

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+- Add Zenodo DOI badge to README; upgrade the BibTeX citation block with the concept DOI (`10.5281/zenodo.19646175`) and list author as Isaac Gerber (matching `CITATION.cff`). Add `doi:` and `identifiers:` entries (concept + versioned) to `CITATION.cff`. DOI was minted by Zenodo when v3.1.3 was released.
+
 ## [3.1.3] - 2026-04-18
 
 ### Added

CITATION.cff

@@ -9,6 +9,14 @@ authors:
 license: MIT
 version: "3.1.3"
 date-released: "2026-04-18"
+doi: "10.5281/zenodo.19646175"
+identifiers:
+  - type: doi
+    value: "10.5281/zenodo.19646175"
+    description: "Concept DOI — always resolves to the latest release"
+  - type: doi
+    value: "10.5281/zenodo.19646176"
+    description: "Versioned DOI for v3.1.3"
 url: "https://github.com/igerber/diff-diff"
 repository-code: "https://github.com/igerber/diff-diff"
 keywords:

README.md

@@ -5,6 +5,7 @@
 [![License: MIT](https://img.shields.io/badge/License-MIT-blue.svg)](https://opensource.org/licenses/MIT)
 [![Downloads](https://img.shields.io/pypi/dm/diff-diff.svg)](https://pypi.org/project/diff-diff/)
 [![Documentation](https://readthedocs.org/projects/diff-diff/badge/?version=stable)](https://diff-diff.readthedocs.io/en/stable/)
+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.19646175.svg)](https://doi.org/10.5281/zenodo.19646175)
 
 A Python library for Difference-in-Differences (DiD) causal inference analysis with an sklearn-like API and statsmodels-style outputs.
 
@@ -3067,12 +3068,16 @@ If you use diff-diff in your research, please cite it:
 ```bibtex
 @software{diff_diff,
   title = {diff-diff: Difference-in-Differences Causal Inference for Python},
-  author = {{diff-diff contributors}},
+  author = {Gerber, Isaac},
+  year = {2026},
   url = {https://github.com/igerber/diff-diff},
+  doi = {10.5281/zenodo.19646175},
   license = {MIT},
 }
 ```
 
+The DOI above is the Zenodo concept DOI — it always resolves to the latest release. To cite a specific version, look up its versioned DOI on [the Zenodo project page](https://doi.org/10.5281/zenodo.19646175).
+
 See [`CITATION.cff`](CITATION.cff) for the full citation metadata.
 
 ## License