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Correct served KL calculation for truncated logprobs - #140
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The old calculation summed only shared top-logprob entries and divided by their count. It could report a negative KL score.
Use shared tokens plus the remaining probability mass, then average per position. The output labels this as coarsened KL, a lower bound rather than full-vocabulary KL. Also reject invalid probabilities and nonpositive strides.
13 tests pass, including known cases, an HTTP comparison and 200 randomized truncations. This fixes the evaluator; it does not establish that one quantization recipe beats another.
I used AI assistance and reviewed and tested the changes.