fix: add missing healing_done field to HUDStats

- Add healing_done: Decimal field to track HP healed
- Add to to_dict() for serialization
- Fixes AttributeError in on_heal_event handler

ui/hud_overlay_clean.py

@@ -105,6 +105,7 @@ class HUDStats:
     # Damage stats (optional)
     damage_dealt: Decimal = Decimal('0.0')
     damage_taken: Decimal = Decimal('0.0')
+    healing_done: Decimal = Decimal('0.0')  # HP healed
 
     # Shrapnel (optional)
     shrapnel_total: Decimal = Decimal('0.0')
@@ -153,6 +154,7 @@ class HUDStats:
             'hofs_count': self.hofs_count,
             'damage_dealt': str(self.damage_dealt),
             'damage_taken': str(self.damage_taken),
+            'healing_done': str(self.healing_done),
             'shrapnel_total': str(self.shrapnel_total),
             'loot_other': str(self.loot_other),
             'highest_loot': str(self.highest_loot),