* vulkan: add dedicated iq4_xs mat-vec shader
Dedicated mul_mat_vec_iq4_xs for the dmmv path, replacing the generic fallback. ~+6-17% token generation on RDNA4 depending on model.
Assisted-by: Pi agent with Qwen3.8 27B
* vulkan iq4_xs: remove dead n_it unroll branch
Remove the n_it <= 8 experimental branch that attempted to fully unroll
the block loop. Since n_it is a runtime value, [[unroll]] is ignored by
the compiler, making both branches equivalent. Kept the simple loop
matching mul_mat_vec_iq3_s.comp.
WakeUpMorty
df750f76bb