Cohort Retention Table Stats Block for React

Edit COHORTS at the top of the file: a name, the cohort's size and a kept list of percents for month 1 and on (month 0 is always 100). A newer cohort has a shorter list, which is what makes the triangle. The averages are the mean of the cohorts that have reached each month, rounded to a percent, and the headline reads month one's average.

A cell's step comes from its percent: under 20, 40, 60, 80, and 80 or more. They are solid mixes of the foreground colour into the card, and the text is the foreground colour up to the third step and the page colour from the fourth, so every cell keeps its contrast in light and dark. The number is always in the cell, so shade is never the only signal.

It needs a caption, column headings and a header for each row, and a native table gives all three on every library. Cohort names are row headings, months are column headings (announced as "Month 3"), and an empty cell says "Not reached yet". The table is forced left to right so months run from the start of the line in every language.

For each month it averages only the cohorts that have a value for it, so M5 is the oldest cohort's figure alone and M1 is the mean of five. It is a simple mean of percents, not weighted by cohort size, and the row is set apart from the cohorts with space and a bold heading.

M4 and M5 are hidden so the remaining columns stay wide enough to read their percents at 320 pixels, and each cohort's size moves under its name. The key stays under the table. Nothing scrolls sideways; on wider screens all six months show.