CrossQuantile

The finite-key quantile against libm's erfc, into the far tail

1/1 passed in 3ms

Test	What it does	Result
test_quantile_matches	From failure probability 0.5 to 1e-30 z_pe and the erfc inversion agree to 1.3e-8 absolute and 1.1e-9 relative, the stated accuracy of the rational fit.	✅ pass