* qwen4exp : build the QSA selection without the kq_mask

* clean

* lang

* qwen4exp : reshape the fused QSA selection to the kq_mask streams

ggml_top_k_mask with a single keep row returns [n_kv, n_tokens], while
build_attn_qsa expects the kq_mask layout [n_kv, n_tokens/n_stream, 1, n_stream].
With several streams the shape assert failed at graph reserve.

* test the QSA optimization

* reword

* update build_fused_top_k_mask instead of handwiring

* simplify

* keep il at the end for consistency