feat: implement Spell Focusing Sash (Closes #22)

Adds the Stygian Abyss-era Spell Focusing Sash with full lifecycle,
equipment stat block, and damage sequence behavior:

- New SpellFocusingSash (BaseMiddleTorso) with Brittle, Mana Increase,
  Defense Chance Increase, +10 Strength Requirement, and 255/255 durability.
- Context-menu enable/disable on the worn sash.
- Sequence damage modifier: -30 -> 0 over six casts, then +2 per cast
  capped at +30 (PvM) or +20 held for five casts (PvP), resetting at the
  peak, on target change, on unequip, on disable, on death, and on logout.
- Centralized eligibility gate (SpellFocusingEligible) and shared
  SpellHelper.Damage hooks, plus a direct hook in PainSpike.
- Brittle is forced on under Core.SA via AOS.IsBrittle.
- Regression tests covering stats/property ordering, enable state
  serialization, context-menu toggle, PvM and PvP sequences, target
  resets, disabled state, and non-eligible spells.

Projects/UOContent/Spells/Base/SpellHelper.cs

@@ -958,6 +958,11 @@ namespace Server.Spells
                 bcFrom?.AlterSpellDamageTo(target, ref damageGiven);
                 bcTarget?.AlterSpellDamageFrom(from, ref damageGiven);
 
+                if (SpellFocusingSash.TryGetDamageOffset(spell, from, target, out var spellFocusingOffset))
+                {
+                    damageGiven = AOS.Scale(damageGiven, 100 + spellFocusingOffset);
+                }
+
                 target.Damage(damageGiven, from);
 
                 bcFrom?.OnDamageSpell(target, damageGiven);
@@ -1026,6 +1031,11 @@ namespace Server.Spells
 
                 bcTarget?.AlterSpellDamageFrom(from, ref dmg);
 
+                if (SpellFocusingSash.TryGetDamageOffset(spell, from, target, out var spellFocusingOffset))
+                {
+                    dmg = AOS.Scale(dmg, 100 + spellFocusingOffset);
+                }
+
                 if (Feint.GetDamageReduction(from, target, out var feintReduction))
                 {
                     // example: 35 damage * 50 / 100 = 17 damage
@@ -1110,6 +1120,11 @@ namespace Server.Spells
                 (m_From as BaseCreature)?.AlterSpellDamageTo(m_Target, ref m_Damage);
                 (m_Target as BaseCreature)?.AlterSpellDamageFrom(m_From, ref m_Damage);
 
+                if (SpellFocusingSash.TryGetDamageOffset(m_Spell, m_From, m_Target, out var spellFocusingOffset))
+                {
+                    m_Damage = AOS.Scale(m_Damage, 100 + spellFocusingOffset);
+                }
+
                 m_Target.Damage(m_Damage);
                 m_Spell?.RemoveDelayedDamageContext(m_Target);
             }