Interface PercentileBandDeclaration

A percentile band (fan chart): a median line with nested bands of quantiles drawn around it, as a forecast's prediction intervals or TensorBoard's distribution dashboard draw them.

A range series says nothing about which quantiles its edges are: the same arearange draws a min-max envelope, a 95% confidence interval and a mean plus or minus one standard deviation. Read undeclared, the median is a line and each band an area of its own, correct about every number and silent about how much of the distribution each band holds. The declaration is written on the median line and names each band's series and its two levels, so the figure becomes one percentile_band layer whose quantiles are the bands' edges and the median, at every x the median draws.

The bands must nest: each one strictly inside the next wider one, every lower below 0.5 and every upper above it. A list that does not nest describes no fan chart and is refused. The band series are absorbed into the layer and become no layer of their own; one that cannot be resolved is reported and left out, and the layer keeps the bands that can.