b11413: 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.