feat(reflect): add reflection-based tree serializer with format-agnostic SerdeBase

[CoraBlack]

Summary

• Add a reflection-driven object serializer to gkit::core::reflect: an object is
  expanded into a format-neutral tree (SerdeStruct/SerdeNode), then rendered to a
  target text format through the pure-virtual SerdeBase interface.
• A Json example (in tests) demonstrates extensibility; Xml/Toml can be added by
  overriding the wrapper/gap hooks.

Changes

• Tree (seralize.{hpp,cpp}): SerdeStruct(ObjectId)/(Value) builds a SerdeNode tree (Object/Array/Map/basic) via ClassInfo::for_each_field + get_field, with recursive nesting. SerdeNode stores format-neutral raw leaf values and exposes get_key/get_val/get_type/get_children; SerdeStruct::root().
• SerdeBase (serdebase.{hpp,cpp}): template method seralize(ObjectId) / seralize(key, Value) builds the tree and renders it through 9 pure-virtual hooks (key_wrapper, kv_connect, container wrappers, element gaps, value_wrapper). Virtual destructor; all hooks = 0 (also resolves the undefined-vtable link error).
• Reflection registry: make_getter/make_setter/deduce_type now support Array/Map object members (previously fell through to Null).
• Build: register seralize.cpp + serdebase.cpp in src/core/CMakeLists.txt.
• Tests: test_serdebase.cpp defines a Json subclass and asserts exact output (incl. nested array/map); test_seralize.cpp covers object-to-tree output. The temporary SerdeStruct::to_string() was removed in favor of SerdeBase.

Verification

test_serdebase and test_seralize both build and pass:
{"num":10,"f_num":20.000000,"str":"Hello my girl","arr":100,200,300,"map" {"map_fnum":200.000000,"map_num":100,"map_str":""}}

Follow-ups

• serdebase.hpp (public) includes the private core/reflect/seralize.hpp; promote the
  tree types to include/gkit/ once SerdeStruct leaves its temporary state.
• Deserialization (deserialize) not implemented yet; SerdeStruct(const Value) remains
  declared but undefined.