fix(attention): map grouped decode heads to KV groups

[taking-lying-flat]

Summary

• map each grouped-decode head tile within its KV group before computing query-head indices
• mask partial tail tiles at the KV-group boundary so they cannot read the next group with the previous KV head
• launch one set of head tiles per KV head
• cover GQA configurations (48, 2), (40, 2), and (80, 4), whose groups span multiple 16-head tiles with a partial tail

Problem

The previous stage-1 mapping partitioned the global query-head axis into consecutive tiles, then inferred the KV head from the tile index. When GROUP > 16 and GROUP % 16 != 0, a tile could cross a KV-group boundary. For example, with 48 query heads and 2 KV heads, query heads 24-31 were assigned to KV head 0 instead of KV head 1.