Fix AGWeakAttribute symbol mangling issue

[Kyle-Ye]

Summary

Fixes the linking error when using AGWeakAttribute as a dictionary key by correcting the symbol mangling from struct (V) to the expected type.

Problem

The symbol was being mangled as sSo15AGWeakAttributeVSH0B5GraphMc (struct) instead of sSo15AGWeakAttributeaSH0B5GraphMc (type alias), causing undefined symbol errors during linking.

Solution

Updated the C header declaration to use AG_SWIFT_STRUCT macro which ensures proper Swift import behavior while maintaining ABI compatibility with the system framework.

Test Plan

• Verified that AGExample_2024 builds and runs successfully
• Confirmed the symbol is now correctly mangled
• Tested that AGWeakAttribute can be used as a dictionary key without link errors

Fixes #178