Summary

• Add set_receive_callback_view(std::span<const uint8_t>) API for zero-copy receive path
• Implement lock-free callback storage using shared_ptr + atomic_load/store
• Update do_read() to prefer span callback (no vector allocation) when set
• Maintain backward compatibility with legacy vector callback

Changes

New API

• tcp_socket::set_receive_callback_view(std::function<void(std::span<const uint8_t>)>)
  • Zero-copy: span points directly into internal read buffer
  • Lifetime contract: span valid only during callback invocation
  • Priority: if both callbacks set, span callback takes precedence

Performance Improvements

• Eliminate per-read std::vector allocation/copy when using span callback
• Remove mutex contention from receive hot path via lock-free callback access
• Reduce allocator overhead and cache misses under high TPS

Files Changed

• src/internal/tcp_socket.h - Add span callback API and lock-free storage
• src/internal/tcp_socket.cpp - Implement new functionality
• include/kcenon/network/internal/tcp_socket.h - Public header sync
• tests/CMakeLists.txt - Add tcp_socket_test target
• tests/unit/tcp_socket_test.cpp - Comprehensive unit tests
• docs/CHANGELOG.md, docs/CHANGELOG_KO.md - Documentation

Test Plan

• New unit tests for span callback functionality
• Legacy vector callback continues to work
• Span callback takes precedence when both set
• Error callback works correctly
• Concurrent callback registration stress test
• Full test suite passes

Run tests:

cmake --build build --target network_tcp_socket_test
./build/bin/network_tcp_socket_test

Closes #316
Part of Epic #315