causal mask

With T new queries over S cached keys, the obvious lower-triangular mask is wrong and still runs without an error. The answer derives the right one from absolute positions and names the library default that gets it wrong.

A 4K window over 32K tokens keeps one eighth of the keys, and the speedup over causal attention is not 8x. The answer counts tiles exactly, classifies which need masking, and explains where the rest of the gap goes.

The attention math is ten lines. The grade comes from where the new keys land, which query head reads which KV head, and the causal mask when a chunk of queries sits at the end of a longer cache.