- change `k_copy_src1_to_contiguous` so that uses a precomputed contiguous mapping where all rows "owned" by an expert are in one slice with a know starts and ends
- switch the `O(n_as * n_routed_rows)` contraption to a counting sort-based procedure with `O(n_as + n_routed_rows)` complexity
* SYCL: reduce allocation overhead during flash attention
* tidy up whitespace
* add a note about the flag
* move ggml_sycl_fattn_* into fattn-buffers.hpp
* refactor implementation into fattn-buffers.cpp
* move new_fattn_kv_buffers back into ggml-sycl.cpp