Un-underpopulate '4's by adding sixfirhy: elec theme, unorthodox timings.

Sixfirhies move in bursts: speed 30, but act only on 4 turns out of 12.
They also get a bonus to moving compared to other actions (move cost 60%).

Damage brand is AF_ELEC -- it's unproportionately dangerous, so the numbers
only look small.

Also, electricity heals them!  It won't bring the back from over the edge
if physical part of damage got them to 0 hp or less, in this case, they'll
explode instead.

Keeping with the long tradition of Crawl's demon names, the credit for this
one goes to timecircuits' cat.

1 files changed, 3 insertions, 1 deletions

diff --git a/crawl-ref/source/mon-act.cc b/crawl-ref/source/mon-act.cc
index 1501d747a8..d930815296 100644
--- a/crawl-ref/source/mon-act.cc
+++ b/crawl-ref/source/mon-act.cc
@@ -1747,7 +1747,9 @@ static void _handle_monster_move(monsters *monster)
 
         _monster_regenerate(monster);
 
-        if (monster->cannot_act())
+        if (monster->cannot_act()
+            || monster->type == MONS_SIXFIRHY // these move only 4 of 12 turns
+               && ++monster->number / 4 % 3 != 2)  // but are not helpless
         {
             monster->speed_increment -= non_move_energy;
             continue;