CoinGrinder and SRD disagree on small change #33352

Both CoinGrinder and SRD create a change output, however they disagree on what change output is too small.

Consider SRD selection, which adds CHANGE_LOWER to the target value and in so doing, prevents a selection that is less than CHANGE_LOWER ref

target_value += CHANGE_LOWER + change_fee;

However, CoinGrinder does not add CHANGE_LOWER ref

const CAmount total_target = selection_target + change_target;

I believe the correct behavior is for CoinGrinder to add CHANGE_LOWER to the total_target, like so:

const CAmount total_target = selection_target + CHANGE_LOWER + change_target;

cc @murchandamus

This is intentional.

Knapsack and CoinGrinder use GenerateChangeTarget to make a randomized minimum change, because these two algorithms minimize the overshoot over the minimum change which led to a fingerprint and a clustering of change outputs of the same value. SRD randomly selects inputs, so it already overshoots the minimum change by a random amount.