Collapses consecutive sibling runs of the same {module, function, arity} in a
call tree.
A run of N ≥ 2 identical subtrees (deep ===) folds losslessly to one node with
count: N. A run that shares a function/arity but differs folds to the richest
occurrence (most direct child calls, counted before those children are folded)
with count: N, varies: true — a lossy summary that keeps the shape of one
real call. Runs of 1 are untouched (no keys added), so a
tree with no repetition is unchanged.
Summary
Functions
Folds consecutive identical / same-function sibling runs, bottom-up.