Add Zenodo DOI auto-minting config for GitHub Releases

Enables Zenodo to mint a concept DOI + per-version DOI on every future
GitHub Release. Zenodo's GitHub App reads `.zenodo.json` at the tagged
commit and publishes a software record.

Changes:
- `.zenodo.json` (new): upload_type=software, MIT license, creators block
  (Gerber, Isaac; ORCID 0009-0009-3275-5591), related_identifiers linking
  to the GitHub source (isSupplementTo) and ReadTheDocs (isDocumentedBy).
- `LICENSE` (new): standard MIT text with single-holder copyright.
- `CITATION.cff`: version 3.1.1 -> 3.1.2, date-released updated to the
  v3.1.2 release date.
- `pyproject.toml`: add `LICENSE` to `[tool.maturin].include` so it lands
  in the sdist tarball (not only in PKG-INFO's License-File metadata).

Manual one-time setup (done out-of-band): Zenodo account linked to GitHub,
the igerber/diff-diff repo toggled ON at
https://zenodo.org/account/settings/github/. The next release tag after
v3.1.2 will trigger the Zenodo webhook and mint DOIs.

A follow-up PR will add the concept-DOI badge to README and a `doi:`
field to CITATION.cff once the first DOI has been minted.

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

Author: igerber

.zenodo.json

@@ -0,0 +1,36 @@
+{
+  "upload_type": "software",
+  "title": "diff-diff: Difference-in-Differences Causal Inference for Python",
+  "description": "A Python library for Difference-in-Differences (DiD) causal inference analysis. Provides sklearn-like estimators for modern DiD methods including Callaway-Sant'Anna, Synthetic DiD, Honest DiD, event studies, and parallel trends testing. Validated against R packages (did, synthdid, fixest).",
+  "creators": [
+    {
+      "name": "Gerber, Isaac",
+      "orcid": "0009-0009-3275-5591"
+    }
+  ],
+  "license": "MIT",
+  "access_right": "open",
+  "keywords": [
+    "difference-in-differences",
+    "causal-inference",
+    "econometrics",
+    "python",
+    "treatment-effects",
+    "event-study",
+    "staggered-adoption",
+    "parallel-trends",
+    "synthetic-control",
+    "panel-data"
+  ],
+  "related_identifiers": [
+    {
+      "relation": "isSupplementTo",
+      "identifier": "https://github.com/igerber/diff-diff",
+      "resource_type": "software"
+    },
+    {
+      "relation": "isDocumentedBy",
+      "identifier": "https://diff-diff.readthedocs.io/en/stable/"
+    }
+  ]
+}

CITATION.cff

@@ -7,8 +7,8 @@ authors:
     family-names: Gerber
     orcid: "https://orcid.org/0009-0009-3275-5591"
 license: MIT
-version: "3.1.1"
-date-released: "2026-04-16"
+version: "3.1.2"
+date-released: "2026-04-18"
 url: "https://github.com/igerber/diff-diff"
 repository-code: "https://github.com/igerber/diff-diff"
 keywords:

LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2025-2026 Isaac Gerber
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

pyproject.toml

@@ -95,7 +95,7 @@ manifest-path = "rust/Cargo.toml"
 python-packages = ["diff_diff"]
 # Bundle LLM guide files (auto-inclusion varies across maturin 1.4-2.0; pin explicitly).
 # Bare-string glob form includes in both sdist and wheel.
-include = ["diff_diff/guides/*.txt"]
+include = ["diff_diff/guides/*.txt", "LICENSE"]
 
 [tool.pytest.ini_options]
 testpaths = ["tests"]