A reproducibly simulated seasonal monthly time series in tidy long form, for
the time-series examples. It holds two product lines (indoor and
outdoor), each with six years of monthly observations carrying a linear
trend, a clear twelve-month seasonal cycle (with different seasonal peaks per
line) and noise. The two-series shape exercises the multi-group
timeseries_plot() path on first-party data; extracting one series as a
monthly ts (frequency 12) lets decompose_plot() and acf_plot() reveal
the trend and seasonality.