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/player.js

@@ -494,7 +494,7 @@ const mech = {
         let dmg = 1
         dmg *= mech.fieldHarmReduction
         if (tech.isSpeedHarm) dmg *= 1 - Math.min(player.speed * 0.0185, 0.55)
-        if (tech.isSlowFPS) dmg *= 0.85
+        if (tech.isSlowFPS) dmg *= 0.8
         if (tech.isPiezo) dmg *= 0.85
         if (tech.isHarmReduce && mech.fieldUpgrades[mech.fieldMode].name === "negative mass field" && mech.isFieldActive) dmg *= 0.6
         if (tech.isBotArmor) dmg *= 0.97 ** tech.totalBots()