Added test_seasonality() to test for seasonality at a specified frequency using the autocorrelation-based procedure from the M4 Forecasting Competition.
Added forecast_naive2() to generate Naive2 benchmark forecasts using conditional seasonal adjustment.
Added NAIVE2() and supporting S3 methods to integrate the Naive2 benchmark with the fabletools model interface, including forecast distributions, fitted values and residuals.
Expanded the package documentation with references covering time series cross-validation, forecast accuracy measures, and benchmark forecasting methods.
