feat(c++): support customize c++ field meta

[chaokunyang]

Why?

Currently, Fory's C++ xlang serialization treats all struct fields uniformly:

1. Null checks are always performed - Even for fields that are never null, Fory writes a null/ref flag (1 byte per field)
2. Reference tracking is always applied (when enabled globally) - Even for fields that won't be shared/cyclic
3. Field names use meta string encoding - In schema evolution mode, field names consume space

These defaults ensure correctness but introduce unnecessary overhead when the developer has specific knowledge about their data model.

What does this PR do?

Implements fory::field<> template for compile-time field metadata, enabling performance and space optimization:

Core API

namespace fory {
  struct nullable {};  // Mark shared_ptr/unique_ptr as nullable
  struct not_null {};  // Explicit non-null (for future pointer types)
  struct ref {};       // Enable reference tracking (shared_ptr only)

  template <typename T, int16_t Id, typename... Options>
  class field { /* ... */ };
}

Type Rules

Type	Default Nullable	Default Ref	Allowed Options
Primitives, std::string	false	false	None
std::optional<T>	true (inherent)	false	None
std::shared_ptr<T>	false	false	nullable, not_null, ref
std::unique_ptr<T>	false	false	nullable only

Usage Example

struct Document {
  fory::field<std::string, 0> title;
  fory::field<int32_t, 1> version;
  fory::field<std::optional<std::string>, 2> description;
  fory::field<std::shared_ptr<User>, 3> author;                        // non-nullable
  fory::field<std::shared_ptr<User>, 4, fory::nullable> reviewer;      // nullable
  fory::field<std::shared_ptr<Node>, 5, fory::ref> parent;             // ref tracking
  fory::field<std::shared_ptr<Node>, 6, fory::nullable, fory::ref> p;  // both
};

FORY_STRUCT(Document, title, version, description, author, reviewer, parent, p);

Compile-Time Validation

// ✅ Valid
fory::field<std::string, 0> name;
fory::field<std::shared_ptr<Node>, 1, fory::ref> parent;

// ❌ Compile errors (static_assert)
fory::field<int32_t, 0, fory::nullable> age;           // nullable on primitive
fory::field<std::unique_ptr<T>, 1, fory::ref> ptr;     // ref on unique_ptr

Files Changed

New Files:

• cpp/fory/meta/field.h - Core field template and type traits
• cpp/fory/meta/field_test.cc - 22 unit tests
• cpp/fory/serialization/field_serializer_test.cc - 20 serialization tests

Modified Files:

• cpp/fory/meta/BUILD, CMakeLists.txt - Build targets
• cpp/fory/serialization/BUILD, CMakeLists.txt - Build targets
• cpp/fory/serialization/struct_serializer.h - Field metadata integration
• cpp/fory/serialization/type_resolver.h - Field type unwrapping

Related issues

Closes #3003

Related: #2906, #1017

Does this PR introduce any user-facing change?

• Does this PR introduce any public API change?
  • New fory::field<> template for field metadata
  • New tag types: fory::nullable, fory::not_null, fory::ref
• Does this PR introduce any binary protocol compatibility change?
  • No, the wire format is unchanged

Benchmark

For a struct with 10 fields using fory::field<> with non-nullable defaults:

• Space savings: ~20 bytes per object (eliminates null flags)
• CPU savings: Fewer ref tracking operations when not needed
• Zero runtime overhead: All metadata is compile-time only