Bump version to 2.1.9

[igerber]

Summary

• Bump version from 2.1.8 to 2.1.9 in __init__.py, pyproject.toml, and rust/Cargo.toml
• Add CHANGELOG entry for v2.1.9 documenting PR Add unified LOOCV for TROP joint method with Rust acceleration #113 changes:
  • Unified LOOCV for TROP joint method with Rust acceleration
  • TROP joint method Rust/Python parity fixes (valid_count bug, NaN handling, jackknife fixes)

Methodology references (required if estimator / math changes)

• Method name(s): N/A - version bump only, no methodology changes
• 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

---

Generated with Claude Code

[github-actions]

Overall assessment: ⚠️ Needs changes

Executive Summary

• Changelog for 2.1.9 lists substantive TROP LOOCV/variance behavior changes; ensure those changes are already merged and documented, or adjust the release notes to match this PR’s contents.
• Reviewed docs/methodology/REGISTRY.md; no estimator or inference code changes in this diff to verify against registry requirements.
• Version bumps are consistent across diff_diff/__init__.py, pyproject.toml, and rust/Cargo.toml.

Methodology

• Severity: P2 | Impact: Release notes assert TROP estimator, variance/SE, and assumption handling changes, but this PR contains only version/changelog edits, so methodology adherence cannot be verified and the release notes may be misleading if those changes are not already merged | Concrete fix: Confirm the referenced changes are already in the codebase and aligned with docs/methodology/REGISTRY.md/TROP docstrings; otherwise update the changelog to reflect the actual contents of this release. Location: CHANGELOG.md:L8-L23

Code Quality

• No findings.

Performance

• No findings.

Maintainability

• No findings.

Tech Debt

• No findings.

Security

• No findings.

Documentation/Tests

• No findings.