Document that percentile helpers return quiet NaNs for NaN-containing inputs.
Make quartile expected-value tests compute ranks from the documented
round(p / 100 * (n - 1)) rule instead of reusing statistics::percentile_rank(),
so rank regressions are caught independently.
Extend timeout-warning coverage to exercise the too-few-samples max-noise path
in addition to unavailable, invalid, and infinite stdev-noise inputs.
d230a16e2b