Stream a column selection (and an optional ascending row selection) of a
sparse backend into a fresh backend without materialising the survivors.
Shared by the split / subsample CLI handlers and by downstream crates
(faba’s qc) that filter a matrix by cells and features.
Cell-axis quality control (library-only): robust (MAD) outlier detection
plus the near-empty floor. Built on the streaming stat collectors in the
sibling qc module and consumed by senna / pinto — NOT by the data-beans
CLI — so it lives in its own module that the binary never compiles (keeps
qc fully bin-used and avoids dead-code in the bin target).