Keep SnapKV window scores on device 🤖🤖🤖 - #316

PR description

Keep SnapKV's observation-window sentinel on the device instead of reading scores.max().item() back to the host. This removes that host synchronization while retaining the global maximum plus one and the existing score shape.

Validation

• Targeted score and CUDA synchronization tests: 7 passed each on B300 and two GH200 systems.
• Compared a fixed-weight, randomly initialized small Llama before and after this change: FP16/BF16 with eager and SDPA attention produced identical prefill logits, compressed keys and values, and one decode step. This does not establish pretrained-model accuracy or end-to-end performance.
• make style passed; the full make test suite was not run.

Checklist

• Tests are working (make test) — targeted tests passed; full suite not run
• Code is formatted correctly (make style)
• Copyright header is included
• All commits are signed off

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