SDPA fallback now respects sliding window during single-token KV-cache
decode by slicing K/V to the last (window + 1) tokens.
Also simplifies the mask building for chunk inference to properly apply
sliding window in that path as well.
Fixes#452
Co-Authored-By: Kartik Vashishta <kartikv776@gmail.com>
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>