Add USE_VENDORED_JSON option (fixes #959)

[facontidavide]

Summary

• Adds USE_VENDORED_JSON CMake option (default ON) so users can opt out of the bundled nlohmann::json, fixing ODR violations (Version conflict with embedded nlohmann::json can cause undefined behavior #959)
• When OFF, links against nlohmann_json::nlohmann_json and uses system headers
• Downstream consumers get find_dependency via exported CMake config

Test plan

• Default build (USE_VENDORED_JSON=ON): 308/308 tests pass
• System build (USE_VENDORED_JSON=OFF): 308/308 tests pass

Generated with Claude Code

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feature/use-vendored-json

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.90%. Comparing base (53040fa) to head (a58856c).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1080   +/-   ##
=======================================
  Coverage   65.90%   65.90%           
=======================================
  Files         225      225           
  Lines       12727    12727           
  Branches     1186     1186           
=======================================
  Hits         8388     8388           
  Misses       4289     4289           
  Partials       50       50           

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