Why (cost, same concern as Adolanium's line-matching note): pairing repeated names by body is
all-pairs within a group, and the per-group lookup of base units scanned the whole base file,
so a codemod touching every function of a big file was quadratic (5,000 edited unique names:
5.4 s -> 0.04 s with a per-file name index). A group whose all-pairs check would exceed
250,000 pairs (e.g. hundreds of edited `it(...)` callbacks in one scope) falls back to the
ordinal pairing, which is bounded; measured 2,000-callback group: 0.05 s.
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