feat: add native C++ unit tests for state tracking logic

[tomkp]

Summary

Adds native C++ unit tests for the Issue #111 state tracking logic using Catch2.

• Extracts state detection logic into pure function DetectCardStateChange
• Adds Catch2 test framework (header-only, v2.13.10)
• Adds 6 unit tests covering all state transitions
• Refactors reader_monitor.cpp to use the extracted function
• Adds CI step to run native tests on all platforms

Closes #119

Changes

New files:

• src/reader_state_utils.h - Pure function for card state detection
• src/test/catch.hpp - Catch2 test framework
• src/test/test_main.cpp - Catch2 main entry point
• src/test/reader_state_utils_test.cpp - Unit tests

Modified files:

• binding.gyp - Added test target
• package.json - Added test:native script
• src/reader_monitor.cpp - Refactored to use DetectCardStateChange
• .github/workflows/test.yml - Added native test CI step

Test plan

• All 6 native C++ tests pass
• All 106 TypeScript unit tests pass
• Build succeeds locally
• CI passes on all platforms (Ubuntu, macOS, Windows)

[tomkp]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.