feat(c++): add SharedWeak<T> for circular reference support

[chaokunyang]

Why?

C++ serialization needs support for weak pointers to handle circular references in graph-like structures (e.g., parent-child relationships). Unlike std::weak_ptr<T>, we need a wrapper that allows updating the internal pointer during deserialization for forward reference resolution.

What does this PR do?

Adds SharedWeak<T> - a serializable wrapper around std::weak_ptr<T> designed for graph structures with circular references.

Key features:

1. SharedWeak<T> wrapper class (weak_ptr_serializer.h)

   • Stores std::weak_ptr<T> inside std::shared_ptr<Inner> so all copies share the same internal storage
   • Enables forward reference resolution via callbacks during deserialization
   • Provides from(), upgrade(), update(), expired() methods

2. Forward reference resolution (ref_resolver.h)

   • Added add_update_callback(ref_id, UpdateCallback) overload to RefReader
   • When deserializing a weak pointer that references an object not yet created, registers a callback to update it later

3. Serializer implementation (weak_ptr_serializer.h)

   • Handles NULL_FLAG, REF_FLAG, and REF_VALUE_FLAG properly
   • Requires track_ref=true for reference tracking
   • Supports forward reference resolution via callbacks

4. Type traits

   • is_shared_weak<T> - detection trait
   • requires_ref_metadata<SharedWeak<T>> - always true
   • is_nullable<SharedWeak<T>> - always true

Example usage:

struct Node {
  int32_t value;
  SharedWeak<Node> parent;              // Non-owning back-reference
  std::vector<std::shared_ptr<Node>> children;  // Owning references
};
FORY_STRUCT(Node, value, parent, children);

auto parent = std::make_shared<Node>();
parent->value = 1;

auto child = std::make_shared<Node>();
child->value = 2;
child->parent = SharedWeak<Node>::from(parent);
parent->children.push_back(child);

// Serialize and deserialize
auto bytes = fory.serialize(parent);
auto result = fory.deserialize<std::shared_ptr<Node>>(bytes);

// Parent reference is preserved
assert(result->children[0]->parent.upgrade() == result);

Related issues

#1017
#2906
Closes #2976
Closes #2977

Does this PR introduce any user-facing change?

• Does this PR introduce any public API change?
  • Adds new SharedWeak<T> class for C++ users
• Does this PR introduce any binary protocol compatibility change?

Benchmark

N/A - this is a new feature addition.