Bump version to 2.3.1

[igerber]

Summary

• Bump version from 2.3.0 to 2.3.1 across all version files
• Convert CHANGELOG [Unreleased] section to [2.3.1] - 2026-02-15
• Add comparison link for v2.3.0...v2.3.1

Methodology references (required if estimator / math changes)

• N/A — no methodology changes

Validation

• Tests added/updated: No test changes (version-only update)
• Backtest / simulation / notebook evidence: 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

• No estimator, math, or inference changes detected; methodology registry cross-check not applicable for a version-only bump.
• Version numbers updated consistently across Python, Rust, and packaging metadata.
• Changelog correctly finalizes [Unreleased] into a dated release section and adds a comparison link.
• No tests or documentation changes required for a version-only update.

Methodology

• No findings. (Version-only changes; no estimator or inference logic touched.)

Code Quality

• No findings. (Changes limited to version strings and changelog.)

Performance

• No findings. (No runtime code changes.)

Maintainability

• No findings. (Version alignment improves consistency.)

Tech Debt

• No findings.

Security

• No findings. (No credentials or secrets introduced.)

Documentation/Tests

• No findings. (Changelog updated; no tests expected for version bump.)