feat(config): add small_vec feature flag with experiment guard

Add conditional compilation logic for WHEEL_SMALL_VEC feature that
requires WHEEL_EXPERIMENT to be defined. Provides compile-time error
when small_vec is used without the experimental flag enabled.

Author: Reim-developer

wheel_memory/include/wheel_memory/config.hxx

@@ -5,6 +5,15 @@
 #define WHEEL_MEMORY_END_NAMESPACE }
 #define WHEEL_MEMORY_NAMESPACE_NAME wheel_memory
 
+#if defined (WHEEL_EXPERIMENT) && defined(WHEEL_SMALL_VEC)
+    #define HAS_USE_SMALL_VEC 1
+
+#elif !defined(WHEEL_EXPERIMENT) && defined(WHEEL_SMALL_VEC)
+    #error "Cannot use WHEEL_SMALL_VEC without WHEEL_EXPERIMENT"
+#else 
+    #define HAS_USEHAS_USE_SMALL_VEC 0
+#endif 
+
 #if defined(_MSC_VER)
     #define WHEEL_NO_INLINE __declspec(noinline)