Add JSON output to the validator

[bsriramprasad]

Summary

• add opt-in validator --json machine-readable output
• auto-detect H.264 and H.265 codecs in JSON mode when -c is omitted
• preserve complete framework report data, including provenance, counters, latest validation details, and exact timestamps
• emit structured JSON errors with nonzero exit codes
• add serializer tests and Meson/Visual Studio project integration

Compatibility

The existing human-readable validator behavior remains the default. JSON mode does not write validation_results.txt.

Validation

• clean native build
• framework unit tests
• JSON serializer tests
• JSON argument/runtime error smoke tests
• formatting and XML validation

Windows project files were updated but not built on macOS.