Bump version to 2.1.6

[igerber]

Summary

• Bump version 2.1.5 → 2.1.6 across all locations
• Add CHANGELOG entry documenting methodology verification updates from PR Update REGISTRY.md with verified DifferenceInDifferences behavior #106

Methodology references (required if estimator / math changes)

• N/A - version bump only, no methodology changes

Validation

• Tests added/updated: No test changes (version bump only)
• Backtest / simulation / notebook evidence (if applicable): N/A

Security / privacy

• Confirm no secrets/PII in this PR: Yes

Files changed

• CHANGELOG.md - Added [2.1.6] entry and comparison link
• diff_diff/__init__.py - Updated __version__
• pyproject.toml - Updated version
• rust/Cargo.toml - Updated version

Next steps after merge

git tag v2.1.6
git push --tags

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Generated with Claude Code

[github-actions]

Overall Assessment

• ✅ Looks good

Executive Summary

• No estimator/math/SE changes in this diff; methodology registry cross-check not triggered.
• Version bumped consistently across Python and Rust metadata (diff_diff/__init__.py#L139, pyproject.toml#L7, rust/Cargo.toml#L3).
• Changelog entry for 2.1.6 added; ensure it matches actual release contents (CHANGELOG.md#L8).
• No tests updated in this PR; residual risk is release-note accuracy for methodology verification claims.

Methodology

• P3 (Info) — Impact: No estimator, identification, weighting, or variance changes in diff. Concrete fix: None.

Code Quality

• P3 (Info) — Impact: No code path changes to assess. Concrete fix: None.

Performance

• P3 (Info) — Impact: No runtime-impacting changes. Concrete fix: None.

Maintainability

• P3 (Info) — Impact: No maintainability-impacting changes. Concrete fix: None.

Tech Debt

• P3 (Info) — Impact: No tech-debt changes. Concrete fix: None.

Security

• P3 (Info) — Impact: No security-relevant changes. Concrete fix: None.

Documentation/Tests

• P3 — Impact: Release notes could mislead if the listed methodology tests/REGISTRY updates are not actually part of 2.1.6. Concrete fix: Confirm those items are in the release history or adjust the entry at CHANGELOG.md#L10 and CHANGELOG.md#L16.

Open Questions

• Assumed the 2.1.6 changelog summarizes prior merged work rather than changes in this PR.

Change Summary

• Version bumped to 2.1.6 in Python and Rust metadata (diff_diff/__init__.py#L139, pyproject.toml#L7, rust/Cargo.toml#L3).
• Changelog entry and compare link added (CHANGELOG.md#L8, CHANGELOG.md#L533).