score_memories() drops each memory from the context one at a time,
runs prompt_logprobs against the full conversation, and builds a
divergence matrix: memories × responses.
Row sums = memory importance (for graph weight updates)
Column sums = response memory-dependence (training candidates)
Uses vLLM's prompt_logprobs to check "would the model have said
this without this memory?" — one forward pass per memory, all
responses scored at once. ~3s per memory on B200.
Co-Authored-By: Proof of Concept <poc@bcachefs.org>
