Flagging that this is doing TTT during Val but compliantly. @0hq

I believe these make it allowed:

1. No training before evaluation: Each chunk is evaluated first, loss is recorded, then training occurs
2. No re-evaluation: Tokens are scored exactly once; training on chunk N cannot affect scores for chunks 0..N
3. No multiple passes: The validation set is processed in a single sequential pass (32 chunks)