fix(armor): add decay_per_hp field and ClassVar constants to ArmorPiece

- Added decay_per_hp field for custom economy per piece
- Added BASE_DECAY_FACTOR and MAX_DURABILITY as ClassVar
- Added ClassVar to typing imports

core/armor_system.py

@@ -13,7 +13,7 @@ Implements Entropia Universe armor mechanics:
 
 from dataclasses import dataclass, field
 from decimal import Decimal
-from typing import Optional, Dict, List, Tuple
+from typing import Optional, Dict, List, Tuple, ClassVar
 from enum import Enum, auto
 
 
@@ -232,14 +232,15 @@ class ArmorPiece:
     protection: ProtectionProfile = field(default_factory=ProtectionProfile)
     durability: int = 2000  # Durability affects economy
     weight: Decimal = Decimal("1.0")  # Weight in kg
-
-    # Base decay factor: 0.05 PEC per HP (20 hp/pec standard)
-    BASE_DECAY_FACTOR: Decimal = Decimal("0.05")
-    MAX_DURABILITY: int = 100000
+    decay_per_hp: Decimal = Decimal("0.05")  # PEC per HP absorbed (0.05 = 20 hp/pec)
 
     # Optional plate attachment
     attached_plate: Optional[ArmorPlate] = None
 
+    # Class constants (not instance fields)
+    BASE_DECAY_FACTOR: ClassVar[Decimal] = Decimal("0.05")
+    MAX_DURABILITY: ClassVar[int] = 100000
+
     def get_base_protection(self) -> ProtectionProfile:
         """Get base protection without plate."""
         return self.protection