nail gun rework

damage immunity graphic is more obvious
difficulty mode no longer increases after killing final boss (it was too confusing)

tech: Higgs mechanism - you can't move when firing, reduce harm by 60% when firing
  removed Galilean group

removed gun flechettes
removed critical bifrication
removed railroad ties
removed incendiary (for nail gun)

nail gun:
  needle gun: accurate, piercing mobs, fire 3 at a time
  rivet gun: accurate, ballistic arc, doesn't use much ammo
    tech: bigger rivets (9x stack)
  all nails
    radioactive 2s tick (about 70% damage increase)
      0.5s full tick
      4s longer tick
    critical hits -> explosion (from flechettes)

js/engine.js

@@ -108,7 +108,7 @@ function collisionChecks(event) {
                             return
                         }
                         mech.damage(dmg);
-                        if (tech.isPiezo) mech.energy += 2;
+                        if (tech.isPiezo) mech.energy += 4;
                         if (tech.isBayesian) powerUps.ejectTech()
                         if (mob[k].onHit) mob[k].onHit(k);
                         mech.immuneCycle = mech.cycle + tech.collisionImmuneCycles; //player is immune to collision damage for 30 cycles