coinage-layer task #87: 4-coin exact subset-sum with sharp None

find_four_coin_exact_cover: quadruple-nested loop with four-level
accumulator pattern. Same structural shape as 2/3-coin, one more
dimension. The pattern scales — N-coin extension is mechanical
proportional growth with the same proof skeleton.

Sharp None postcondition over all quadruples of pairwise-distinct
Vec indices.

258 verified, 0 errors. Task #87 progress: 1/2/3/4-coin all ship with
sharp None.

Author: TorstenStueber