Decode grid (num_kv_heads, num_seqs) assumes each seq has <= kBlockQ
tokens. For mixed batches (decode + prefill), avg_q is low but some
seqs have hundreds of tokens, causing truncation. Added max_seqlen_q
to args and check it in select_tile_tier to force medium tier (1D
grid with Q tile iteration) for mixed batches.
362/362 no-window shapes now pass.
Made-with: Cursor