kv offload

Recomputing a prefix costs FLOPs proportional to the model, fetching it costs bytes proportional to the cache. One ratio decides it, and the answer shows by how much fetching wins and the three situations where it still loses.

Each tier is worth exactly the recompute it avoids minus the fetch it costs. Work that out per token and the design writes itself, including the tier that should not exist for small models and the SSD that wears out in a week.