llama : build the k-pool selection in the causal mask layout (#30480)
* llama : build the k-pool selection in the causal mask layout
- the glm5-next indexer folded the causal mask in through a [n_kv, n_tokens]
view: one reshape of the mask ahead of the add, and one of the result ahead
of every layer reading the selection, including the shared-indexer layers
- build the selection directly in the mask layout, as qwen4exp already does,
and drop the now-degenerate reshape in its attention as well
- assert the selection width matches the input it is taken from
Assisted-by: pi:llama.cpp/Qwen3.8-Flash-Next
* cont : remove comment