The callosum reads only each store's rank ordering - never its raw scores - and fuses with
Reciprocal Rank Fusion: weight / (k + rank). Because it ignores magnitudes,
the merge survives any store swapping its embedder. Per-store weight is the only cross-store trust lever;
per-store floor guards against rank-1-of-garbage. A slow or down store is skipped for that call - it degrades the
result, it never takes the fan down.