ann-backends

Every method in the single-cell vertical starts by building a kNN graph, and an approximate backend makes that step non-deterministic in ways that propagate into every downstream embedding. Pinning both hnswlib and pynndescent is what closes a reproducibility gap in the TopoMetry workflows rather than leaving the backend to whatever happens to be installed.

Two conda-forge packages in one fixture, locked green. Pinning a pair of alternatives for a single pipeline slot is deliberate — it is also why this grades L3 rather than the L4 either package would score alone.