Core Distances
For each point and each usable neighborhood level, Core-SG stores core-distance
candidates in core_distances_.
get_core_distance(k) selects column k - 1:
core_k = core.get_core_distance(k=10)
Core distances are part of the mutual-reachability formula and therefore directly affect MST edge weights and hierarchy structure.
In algorithm="score-sg", these values come from approximate neighbor
distances returned by PyNNDescent.