Bump version to 2.4.0

[igerber]

Summary

• Bump version from 2.3.2 to 2.4.0 across all version files
• Add CHANGELOG entry for v2.4.0 documenting Two-Stage DiD estimator (Gardner 2022), Proposition 5 detection, and related fixes
• Add comparison link for v2.4.0

Methodology references (required if estimator / math changes)

• N/A — version bump only, no methodology changes in this PR

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

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Generated with Claude Code

[github-actions]

Overall assessment: ✅ Looks good

Executive Summary

• Version bumped consistently across Python package metadata, the Python module, and the Rust crate. (pyproject.toml:7, diff_diff/__init__.py:151, rust/Cargo.toml:3)
• Changelog adds a 2.4.0 entry describing TwoStageDiD, Prop 5 handling, and related fixes; these statements align with the Methodology Registry and existing code, with no estimator/math changes introduced by this PR. (CHANGELOG.md:8)
• No new code paths were added, so no new edge-case, inference, or performance risks are introduced by this PR.

Methodology

• No findings. Changelog additions align with docs/methodology/REGISTRY.md; no estimator/variance/assumption changes in this PR.

Code Quality

• No findings.

Performance

• No findings.

Maintainability

• No findings.

Tech Debt

• No findings.

Security

• No findings.

Documentation/Tests

• No findings. No tests changed; acceptable for a version bump/release-notes update.