fix: track live memory via bytesAllocated instead of subtracting freed sizes

The old code subtracted oldSize from totalAllocated during deallocation,
which required knowing the object size at free time. This was unsafe
because it could read a freed object to determine its size. The new
approach tracks bytesAllocated by only adding new allocations and
relying on GC marking to account for freed memory, making it both
correct and faster with less code.

src/wren_core.c

@@ -137,7 +137,7 @@ DEF_NATIVE(list_add)
 DEF_NATIVE(list_clear)
 {
   ObjList* list = AS_LIST(args[0]);
-  wrenReallocate(vm, list->elements, list->capacity * sizeof(Value), 0);
+  wrenReallocate(vm, list->elements, 0, 0);
   list->capacity = 0;
   list->count = 0;
   return NULL_VAL;