fix(cache): remove direct nlohmann_json dependency and use vix::json

fix(cache): make nlohmann_json export-safe and avoid linking fallback…

… target

feat(cache): add usage examples for cache module

v0.2.0 — refactor cache module

v0.1.0 — Cache module

- Standalone HTTP cache engine
- Pluggable stores and policies
- Clean public CMake target (vix::cache)
- Minimal dependency: net