Let trickle channels inc/dec toward 0.

Previously they handled only positive values, but now trickling -10 will
produce a stream of -1s. It should handle mixes of positive and
negative.
Simplify running-total channel to do only running totals.

It will reset to 0 after a timeout.
Add delayed-total channel transformer for combo totals.
Track running and total combo damage with channels.

Running total composes a trickle channel with a running total channel,
so that the running total increases smoothly from 0 to its current max.
Grand total sends the entire combo's damage to a trickle channel after
the combo timeout, and a loop reads from the damage trickle to apply
damage/healing. Thus, the combo damage rises smoothly; there's a delay;
and then the damage is applied to the health bar smoothly.