For K∈{2,4,8,16,32} LLVM proves the index bounds and elides the checks,
so get_unchecked buys nothing on the production K=8 path. K=64 was the
only value that defeated bounds-check elision (one check in the tail
scan) and it was instantiated in tests only — drop it.
block_search: cite the k-ary search paper
Document that kary_search is the 'k-ary search on a sorted array' variant
from Schlegel, Gemulla & Lehner (DaMoN 2009), specialized to a lower-bound.
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