## Test fixtures
set.seed(2026)
.x_stat    <- arima.sim(list(ar = 0.6), n = 80)          # stationary
.x_I1      <- cumsum(rnorm(80))                            # I(1)
.x_short   <- rnorm(15)                                    # short
.x_ar11    <- arima.sim(list(ar = 0.5, ma = 0.3), n = 100) # ARIMA(1,0,1)

## Seasonal fixture: monthly series with a clear seasonal pattern
set.seed(2027)
.x_seas <- ts(as.numeric(arima.sim(list(ar = 0.4), n = 72)) +
                10 * sin(2 * pi * (1:72) / 12),
              frequency = 12)

## xreg fixture
set.seed(2028)
.x_xreg_z <- matrix(rnorm(80), ncol = 1, dimnames = list(NULL, "z"))
.x_xreg_y <- as.numeric(arima.sim(list(ar = 0.5), n = 80)) + 2 * .x_xreg_z[, 1]