Add C interface for Notifly with shared library support

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This PR implements a complete C interface for the Notifly notification center library, enabling usage from C programs through a shared library (DLL/SO). The C interface wraps the existing C++ API while maintaining all functionality and performance.

Key Features

• Shared Library: Builds libnotifly_c.so (Linux) / notifly_c.dll (Windows) via CMake
• Handle-based API: Uses opaque pointers for type safety across the C/C++ boundary
• Function Pointer Callbacks: Replaces C++ std::function with C-compatible function pointers
• Complete Functionality: Supports all core features including sync/async notifications, multiple instances, and observer management
• Thread Safety: Inherits thread safety from the underlying C++ implementation

API Overview

// Instance management
notifly_handle notifly_default(void);
notifly_handle notifly_create(void);
void notifly_destroy(notifly_handle handle);

// Observer management  
int notifly_add_observer(notifly_handle handle, int notification_id, 
                        notifly_callback callback, void* user_data);
int notifly_remove_observer(notifly_handle handle, int observer_id);

// Notification posting
int notifly_post_notification(notifly_handle handle, int notification_id, void* data);
int notifly_post_notification_async(notifly_handle handle, int notification_id, void* data);

Usage Example

#include "notifly_c.h"

void sensor_callback(int notification_id, void* data, void* user_data) {
    printf("Sensor data received: notification %d\n", notification_id);
}

int main() {
    notifly_handle notifly = notifly_default();
    int observer_id = notifly_add_observer(notifly, 1001, sensor_callback, NULL);
    notifly_post_notification(notifly, 1001, &sensor_data);
    notifly_remove_observer(notifly, observer_id);
    return 0;
}

Files Added

• include/notifly_c.h - C API header with complete interface definitions
• src/notifly_c.cpp - C++ implementation wrapping the existing API
• test/test_c_interface.c - Comprehensive test suite covering all functionality
• example/c_example.c - Practical sensor monitoring example
• docs/C_INTERFACE.md - Complete API documentation and usage guide

Validation

• All existing C++ tests continue to pass (26/26) ✅
• Complete C interface test suite passes (5/5) ✅
• Both C++ and C examples demonstrate correct functionality ✅
• Shared library has minimal dependencies and proper linking ✅

The implementation maintains backward compatibility while providing a clean, efficient C interface that preserves all the power and features of the original C++ API.

Fixes #5.

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