Overall assessment: ✅ Looks good

Executive summary

• Version bumped to 2.2.1 across Python and Rust metadata (diff_diff/__init__.py:L139, pyproject.toml:L5-L8, rust/Cargo.toml:L1-L4).
• Changelog promoted Unreleased to 2.2.1 dated 2026-02-07 and added compare link (CHANGELOG.md:L8-L58, CHANGELOG.md:L661-L662).
• No estimator, math, inference, or default-behavior code changes detected; methodology registry reviewed for scope only (docs/methodology/REGISTRY.md:L1-L200).

Methodology

• P3 (Info) | Impact: No estimator/math/weighting/variance/assumption changes in diff; registry cross-check not required | Fix: None.

Code Quality

• P3 (Info) | Impact: Metadata-only updates; no logic paths touched (diff_diff/__init__.py:L139, pyproject.toml:L5-L8, rust/Cargo.toml:L1-L4) | Fix: None.

Performance

• P3 (Info) | Impact: No runtime code changes in this PR | Fix: None.

Maintainability

• P3 (Info) | Impact: Version numbers kept consistent across Python and Rust packaging files (diff_diff/__init__.py:L139, pyproject.toml:L5-L8, rust/Cargo.toml:L1-L4) | Fix: None.

Tech Debt

• P3 (Info) | Impact: No new debt introduced in metadata-only changes | Fix: None.

Security

• P3 (Info) | Impact: No secrets or security-sensitive code touched | Fix: None.

Documentation/Tests

• P3 (Info) | Impact: Changelog updated for 2.2.1 and compare link added; no tests modified (CHANGELOG.md:L8-L58, CHANGELOG.md:L661-L662) | Fix: None.