fix: Fixes poison field duration, cleans up skill calculations and poison calculations (#1582)

### Summary
- [X] Fixes poison field duration
- [X] Adds SA+ poison spell calculations
- [X] Cleans up skill calculations
- [X] Adjusts poison level thresholds to properly reflect OSI

Projects/UOContent/Items/Skill Items/Magical/Potions/BasePotion.cs

@@ -195,7 +195,7 @@ public abstract partial class BasePotion : Item, ICraftable, ICommodity
     public static int EnhancePotions(Mobile m)
     {
         var EP = AosAttributes.GetValue(m, AosAttribute.EnhancePotions);
-        var skillBonus = m.Skills.Alchemy.Fixed / 330 * 10;
+        var skillBonus = (int)(m.Skills.Alchemy.Value * 10 / 33);
 
         if (Core.ML && EP > 50 && m.AccessLevel <= AccessLevel.Player)
         {

Projects/UOContent/Items/Weapons/Abilities/InfectiousStrike.cs

@@ -47,7 +47,7 @@ namespace Server.Items
             --weapon.PoisonCharges;
 
             // Infectious strike special move now uses poisoning skill to help determine potency
-            var maxLevel = Math.Max(attacker.Skills.Poisoning.Fixed / 200, 0);
+            var maxLevel = Math.Max((int)(attacker.Skills.Poisoning.Value / 20), 0);
             if (p.Level > maxLevel)
             {
                 p = Poison.GetPoison(maxLevel);

Projects/UOContent/Items/Weapons/Abilities/SerpentArrow.cs

@@ -18,17 +18,15 @@ namespace Server.Items
 
             ClearCurrentAbility(attacker);
 
-            defender.SendLocalizedMessage(1112369); // You have been poisoned by a lethal arrow!
-
             int level;
 
             if (attacker.InRange(defender, 2))
             {
-                level = attacker.Skills.Poisoning.Fixed / 2 switch
+                level = (attacker.Skills.Archery.Value + attacker.Skills.Poisoning.Value) switch
                 {
-                    >= 1000 => 3,
-                    > 850   => 2,
-                    > 650   => 1,
+                    > 199.8 => 3,
+                    > 170.2 => 2,
+                    > 130.2 => 1,
                     _       => 0
                 };
             }
@@ -37,6 +35,7 @@ namespace Server.Items
                 level = 0;
             }
 
+            defender.SendLocalizedMessage(1112369); // You have been poisoned by a lethal arrow!
             defender.ApplyPoison(attacker, Poison.GetPoison(level));
 
             defender.FixedParticles(0x374A, 10, 15, 5021, EffectLayer.Waist);