* metal : optimize argmax and pool_1d

Add a two-pass partial reduction for large-row argmax and use cooperative threadgroup reductions for pool_1d. Allocate scratch space for the argmax partials through the Metal buffer allocator.

Assisted-by: pi:llama.cpp/ggml-org/DeepSeek-V4-Flash-Vision-Exp-GGUF (Mac Studio, M2 Ultra, 192GB)

* tests : add pool_1d perf cases

Add representative POOL_1D max cases for the MoE down-projection amax shapes, reducing the whole intermediate dim.

Assisted-by: pi:llama.cpp/ggml-org/DeepSeek-V4-Flash-Vision-Exp-GGUF (Mac Studio, M2 Ultra, 192GB)

* cont : minor fix