This PR implements complete C API bindings for the KeyboardMonitor functionality, enabling C applications and other language bindings to access cross-platform keyboard monitoring capabilities.

What's Added

C API Header (src/capi/keyboard_monitor_c.h)

• Opaque handle pattern: native_keyboard_monitor_t for safe memory management
• C-compatible enums: native_modifier_key_t with all modifier key flags
• Callback function types: Type-safe callbacks for key events with user data support
• Complete function declarations: Create, destroy, configure, and control keyboard monitoring

C++ Implementation (src/capi/keyboard_monitor_c.cpp)

• Memory-safe wrapper: Proper resource management with RAII patterns
• Exception handling: All C++ exceptions are caught and converted to return codes
• Callback translation: Seamless conversion between C function pointers and C++ std::function
• Modifier key mapping: Accurate translation between C++ and C modifier key enums

Example Application (examples/keyboard_example/)

• Educational demo: Shows all API functions with clear explanations
• Real-world usage: Demonstrates proper initialization, event handling, and cleanup
• Cross-platform testing: Works in both desktop and headless environments
• Signal handling: Graceful shutdown with proper resource cleanup

Key Features

// Simple, intuitive API
native_keyboard_monitor_t* monitor = native_keyboard_monitor_create();
native_keyboard_monitor_set_callbacks(monitor, on_key_pressed, on_key_released, on_modifier_changed, user_data);
native_keyboard_monitor_start(monitor);
// ... handle events ...
native_keyboard_monitor_destroy(monitor);

• Thread-safe: Callbacks execute on monitoring thread, control functions safe from any thread
• Error handling: All functions return success/failure status with null pointer safety
• Platform-agnostic: Works on Linux (X11), macOS (Cocoa), and Windows (Win32)
• Memory efficient: Zero-copy event handling with minimal overhead

Build System Updates

• Added C language support to main CMakeLists.txt for mixed C/C++ compilation
• Integrated keyboard example into the build system
• Maintained backward compatibility with existing build configurations

Testing

The implementation has been verified with:

• ✅ Successful compilation on Linux with GTK/X11 dependencies
• ✅ API functionality testing in headless environment
• ✅ Memory management validation (no leaks in create/destroy cycle)
• ✅ Signal handling and graceful shutdown
• ✅ Integration with existing C++ KeyboardMonitor implementation

This C API follows the same patterns established by the existing display_c.h interface, ensuring consistency across the library's C bindings.

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