test(json): verify every public header compiles independently

[GaspardKirira]

Summary

Add compile tests ensuring each public JSON header includes everything required for independent use.

Current behavior

The module exposes:

• build.hpp;
• convert.hpp;
• dumps.hpp;
• jpath.hpp;
• json.hpp;
• loads.hpp;
• Simple.hpp.

Headers are often included through the umbrella header, which can hide missing direct dependencies.

Expected behavior

Every public header must compile as the first and only Vix JSON include in a translation unit.

Scope

• Add one compile target per public header.
• Compile with GCC.
• Compile with Clang.
• Compile with MSVC.
• Test with strict warnings.
• Test normal literal configuration.
• Test enabled literal configuration.
• Test standalone installation headers.
• Preserve header-only usage.

Suggested files

• modules/json/tests/headers/build_header_test.cpp
• modules/json/tests/headers/convert_header_test.cpp
• modules/json/tests/headers/dumps_header_test.cpp
• modules/json/tests/headers/jpath_header_test.cpp
• modules/json/tests/headers/json_header_test.cpp
• modules/json/tests/headers/loads_header_test.cpp
• modules/json/tests/headers/simple_header_test.cpp
• modules/json/tests/CMakeLists.txt

Acceptance criteria

• Every public header compiles independently.
• Header order does not affect compilation.
• Literal macro configurations compile.
• Installed headers pass the same checks.
• No transitive include is required accidentally.
• Supported compilers pass.

Non-goals

This issue does not include:

• reducing compile time;
• changing public APIs;
• testing private headers;
• module-header generation.