Refactor library with Boost-style conventions, AF_PACKET capture, and C/C++ interfaces

[Copilot]

Refactors the single-header C library into a modern C++11 library using AF_PACKET for packet capture, with both C and C++ interfaces.

Architecture

• Header-only C++ library in include/rawsock/
  • capture.hpp - AF_PACKET based capture with protocol filtering
  • packet.hpp - Network header structures with portable packing macros
  • error.hpp - std::error_code integration
  • config.hpp - Platform detection and compiler compatibility
• C wrapper via rawsock_c.h + src/rawsock_c.cpp
• CMake build system with test/example targets

Key Changes

• Replaced SOCK_RAW with AF_PACKET for layer 2 capture
• Added RAWSOCK_PACKED macro for GCC/Clang/MSVC portability
• Fixed recursive packet filtering → iterative loop (stack overflow risk)
• Shared test utilities in tests/test_common.hpp

Usage

#include <rawsock/rawsock.hpp>

rawsock::capture_config config;
config.interface_name = "eth0";
config.filter_protocol = rawsock::protocol::tcp;

rawsock::capture cap;
cap.open(config);

std::vector<uint8_t> buf(65535);
rawsock::packet_info info;
int n = cap.capture_next(buf.data(), buf.size(), &info);

rawsock_capture_t* cap = rawsock_capture_create();
rawsock_config_t config;
rawsock_config_init(&config);
strcpy(config.interface_name, "eth0");
rawsock_capture_open(cap, &config);

Tests

6 test suites covering packet parsing, error handling, capture, and C interface compatibility.

Original prompt

重构下这个库， 编程风格使用boost，不依赖任何外部库， 使用AF_PACKET 封装网络抓包接口，对外提供c/cpp 接口使用，需要完善的ut测试和文档

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