The positional bias (ape/B) should only modulate the compression
softmax logits (Z + B), NOT be added to the KV content itself.
Paper equation: compressed = softmax(Z + B) · C
Bug was doing: compressed = softmax(Z + B) · (C + B) — poisons every
compressed KV entry with learned positional-bias content.
Fixed in both CSA (compress_csa_reduce_kernel) and HCA
(hca_compress_reduce_kernel) paths in compressor_reduce.cu.