Implement Weak References for Breaking Cycles

Implement weak references in C with a simulated garbage collector.

Requirements:

• Define GCObject with strong_ref_count and a weak_ref_list
• Define WeakRef structure with target pointer and a "living" flag
• Implement gc_create_object() that allocates and registers with GC
• Implement strong_retain() and strong_release() for reference counting
• Implement weak_ref_create() that creates a weak reference (does not increment count)
• Implement weak_ref_deref() that returns target if still alive, NULL if collected
• When strong_ref_count reaches zero, free object and invalidate all weak refs
• Demonstrate a reference cycle broken by weak refs
• Print object lifecycle and weak ref state transitions

Test:

• Show strong ref counting
• Show weak ref creation and deref
• Show cycle broken by weak refs
• Show weak ref invalidated after target freed