Make Entity inherit from Object

Entity gains location from Object and narrows x/y back to int
(entities always have spatial coordinates). No behavioral change —
all existing tests pass unchanged.

src/mudlib/entity.py

@@ -2,14 +2,20 @@
 
 from dataclasses import dataclass, field
 
+from mudlib.object import Object
+
 
 @dataclass
-class Entity:
-    """Base class for anything with position and identity in the world."""
+class Entity(Object):
+    """Base class for anything with position and identity in the world.
 
-    name: str
-    x: int
-    y: int
+    Inherits name, location from Object. Narrows x, y to int (entities
+    always have spatial coordinates).
+    """
+
+    # Entities always have spatial coordinates (override Object's int | None)
+    x: int = 0
+    y: int = 0
     # Combat stats
     pl: float = 100.0  # power level (health and damage multiplier)
     stamina: float = 100.0  # current stamina