feat: add IsSameMillennium comparison helper

[nuxflix]

Description

Fixes #

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that changes existing behavior)
• 📚 Documentation update
• 🔧 Chore (CI, dependencies, refactor)

Checklist

• My code follows the style of this project (go fmt, go vet)
• I have added tests that cover my changes
• All new and existing tests pass: go test -race ./...
• I have added/updated GoDoc comments for all exported functions
• I have added benchmarks for new functions
• I have updated CHANGELOG.md with a description of my changes
• The equivalent date-fns function is referenced (if applicable)

Test Results

go test -race -cover ./...

Notes for Reviewers