Exact-count pruning repairs

The following functions replace the three pruning methods of the supplied TaskGatedPruningManager. They keep the existing manager's masks and training interface, preserve biases, prohibit mask regrowth, and explicitly reject the unimplemented global-allocation option.

The integer convention uses decimal-rational conversion to avoid an unintended binary-floating-point rounding change at an integer count. Stable sorting implements the declared tie rule PyTorchArgsort.