C++: add a lean-header mode that avoids including nlohmann/json.hpp

The C++ renderer currently emits #include <nlohmann/json.hpp> in generated header-like files. Although the generated code is commonly included from many translation units, this pulls the full nlohmann/json implementation into every compile and significantly increases build time.

The existing source-style: multi-source option separates generated types, but the generated headers still depend on the full JSON header. Downstream users have to post-process the output into a forward header, a translation unit, and an implementation include fragment.

Proposal: add a C++ renderer option (for example --lean-header) that emits:

• <name>Fwd.hpp: standard-library dependencies, nlohmann/json_fwd.hpp, generated types, and JSON function declarations.
• <name>Fwd.cpp: includes the forward header and the full nlohmann/json.hpp, then includes the implementation fragment.
• <name>.inc: non-inline JSON helper and serializer definitions.

The default output should remain unchanged. This would make the heavy JSON header a dependency of one implementation translation unit instead of every consumer of the generated types.