Bump version to 2.1.5

[igerber]

Summary

• Bump version from 2.1.4 to 2.1.5 across all version files
• Add CHANGELOG.md entry documenting v2.1.5 release

Methodology references (required if estimator / math changes)

• Method name(s): N/A - version bump only
• Paper / source link(s): N/A
• Any intentional deviations from the source (and why): None

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

Release Checklist

After merge:

• git tag v2.1.5
• git push --tags
• Create GitHub release

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Generated with Claude Code

[github-actions]

Overall assessment

✅ Looks good

Executive summary

• Version/changelog bump only; no estimator/math code changes in this diff.
• CallawaySantAnna base_period and NaN-handling notes in the changelog align with the Methodology Registry and estimator docs (docs/methodology/REGISTRY.md:L165-L223, diff_diff/staggered.py:L212-L286).
• Version numbers are consistent across the Python package and Rust crate (diff_diff/init.py:L134-L138, pyproject.toml:L6-L10, rust/Cargo.toml:L1-L6).

Methodology

• No findings.

Code Quality

• No findings.

Performance

• No findings.

Maintainability

• No findings.

Tech Debt

• No findings.

Security

• No findings.

Documentation/Tests

• Severity: P3 | Impact: No tests updated in this PR; release confidence depends on prior test coverage. | Concrete fix: Confirm the standard test suite for the changes included in 2.1.5 ran before tagging the release.