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

The C++ renderer currently emits `#include ` 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:

- `Fwd.hpp`: standard-library dependencies, `nlohmann/json_fwd.hpp`, generated types, and JSON function declarations.
- `Fwd.cpp`: includes the forward header and the full `nlohmann/json.hpp`, then includes the implementation fragment.
- `.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.

Start with the C++ renderer and its existing source-style: multi-source path, then compare the current generated header and implementation outputs. Define how the new lean-header option produces the named Fwd.hpp, Fwd.cpp, and .inc artifacts while preserving default output; done means the full JSON header is limited to the implementation translation unit.

Written by the indexing model from the issue text.