Release v2.1.4

[igerber]

Summary

• Bump version to 2.1.4 in all locations (__init__.py, pyproject.toml, rust/Cargo.toml)
• Add CHANGELOG entry for v2.1.4 with comparison link

Release Highlights

Added

• Development checklists and workflow improvements in CLAUDE.md
• R-style rank deficiency handling across all estimators with rank_deficient_action parameter

Fixed

• get_params() now includes rank_deficient_action parameter (fixes sklearn cloning)
• NaN vcov fallback in Rust backend for rank-deficient matrices
• MultiPeriodDiD vcov/df computation for rank-deficient designs

Changed

• Rank tolerance aligned with R's lm() default

Methodology references (required if estimator / math changes)

• Method name(s): N/A - version bump only (methodology changes were in previous commits)
• Paper / source link(s): N/A
• Any intentional deviations from the source (and why): N/A

Validation

• Tests added/updated: No test changes in this PR (tests were added in PR Fix rank-deficient matrix handling in OLS solver #91)
• 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]

Methodology

• No findings. No estimator/math/variance/assumption changes in this diff, so registry/paper cross-checks are not applicable.

Code Quality

• No findings.

Performance

• No findings.

Maintainability

• No findings.

Tech Debt

• No findings.

Security

• No findings.

Documentation/Tests

• No findings.

Questions/Assumptions

• Assumption: the methodology changes described in the 2.1.4 changelog are already present in the codebase; this PR is only the release bump.

Overall assessment: ✅ Looks good

Executive Summary

• No code changes; version bumped to 2.1.4 in diff_diff/__init__.py#L139, pyproject.toml#L7, rust/Cargo.toml#L3.
• Changelog updated with the 2.1.4 entry and comparison link in CHANGELOG.md#L8 and CHANGELOG.md#L495.
• Residual risk/testing gap: this PR doesn’t add or modify tests; rely on existing CI coverage for the features referenced in the release notes.