vulkan: sparse flash attention for quantized K/V (#29639)
* vulkan: sparse flash attention for quantized K/V
Assisted-by: Claude
* vulkan: single-scan sparse FA index compaction
The compaction ran one workgroup per mask row and walked the row in
BLOCK_SIZE chunks, with a workgroup scan per chunk. For decode that is
one workgroup doing KV/1024 barrier-bound iterations, so at 128k cells
it cost more than the sparse attention it feeds.
Split the row into contiguous segments instead: one per subgroup with
ballot counting over coalesced loads, or one per thread without
subgroups. A single scan over the segment counts then gives each
segment its output offset. The index list stays ascending.
