build(json): avoid modifying parent CMake configuration globally

[GaspardKirira]

Summary

Prevent the JSON module from unexpectedly changing global CMake variables in parent projects and umbrella builds.

Current behavior

The module sets global variables such as:

• CMAKE_CXX_STANDARD;
• CMAKE_CXX_STANDARD_REQUIRED;
• CMAKE_POSITION_INDEPENDENT_CODE;
• CMAKE_RUNTIME_OUTPUT_DIRECTORY.

A submodule should generally express requirements through its targets rather than altering unrelated parent targets.

Expected behavior

The JSON module should use target-scoped configuration wherever possible.

Standalone build defaults may still be configured without polluting umbrella consumers.

Scope

• Remove unnecessary global C++ standard changes.
• Keep target_compile_features() as the public requirement.
• Review global position-independent-code settings.
• Review runtime output directory changes.
• Scope example output paths locally.
• Scope test output paths locally.
• Add parent-project integration tests.
• Preserve standalone behavior.

Suggested files

• modules/json/CMakeLists.txt
• modules/json/examples/CMakeLists.txt
• modules/json/tests/CMakeLists.txt
• modules/json/benchmarks/CMakeLists.txt

Acceptance criteria

• Adding the module does not change unrelated target standards.
• Adding the module does not redirect unrelated executables.
• vix::json still requires C++20.
• Standalone examples and tests remain easy to locate.
• Umbrella builds remain compatible.
• Parent-project integration tests pass.

Non-goals

This issue does not include:

• changing the required C++ language version;
• redesigning umbrella CMake;
• altering other modules;
• removing standalone support.