Bump version to 2.4.2

[igerber]

Summary

• Bump version from 2.4.1 to 2.4.2 across all version files
• Promote [Unreleased] changelog section to [2.4.2] - 2026-02-18
• Add comparison link for v2.4.2

Methodology references (required if estimator / math changes)

• N/A — no methodology changes (version bump only)

Validation

• Tests added/updated: No test changes
• 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 to 2.4.2 across Python, project metadata, and Rust backend; changelog release header/link updated. Locations: diff_diff/__init__.py:151, pyproject.toml:7, rust/Cargo.toml:3, CHANGELOG.md:8, CHANGELOG.md:774
• No estimator, math, weighting, variance/SE, or default behavior code changes in this diff; methodology registry impact is none. Locations: diff_diff/__init__.py:151, pyproject.toml:7, rust/Cargo.toml:3, CHANGELOG.md:8
• No tests modified in this PR; diff is limited to version/changelog files. Locations: diff_diff/__init__.py:151, pyproject.toml:7, rust/Cargo.toml:3, CHANGELOG.md:8

Methodology
No findings. No methodology-relevant code changed in this diff; registry not impacted.

Code Quality
No findings.

Performance
No findings.

Maintainability
No findings.

Tech Debt
No findings.

Security
No findings.

Documentation/Tests

• Severity: P3 | Impact: Changelog may drift from Keep a Changelog workflow without an Unreleased placeholder, making it easier to miss future entries | Concrete fix: Add an empty ## [Unreleased] section above ## [2.4.2] - 2026-02-18 | Location: CHANGELOG.md:8