feat: add native socket RAII wrapper

[zuudevs]

Summary

Adds the first platform networking primitive for v1: an internal move-only RAII wrapper around native socket handles.

Added

• src/platform/native_socket.hpp
  • internal NativeSocketHandle abstraction
  • Windows SOCKET / POSIX file-descriptor invalid-state handling
  • move-only NativeSocket
  • valid(), get(), release(), reset(), and close()
  • last_socket_error() platform diagnostic helper
• src/platform/native_socket.cpp
  • closesocket() on Windows
  • close() on POSIX
  • deterministic destructor cleanup
• tests/native_socket_test.cpp
  • invalid/default state
  • move construction and move assignment
  • replacement-handle cleanup
  • release() ownership transfer
  • explicit reset/close behavior
  • destructor closes the owned native handle

Build changes

• compile the platform socket implementation into the internal target
• link Ws2_32 only on Windows
• allow internal platform tests to include src/ privately

Architecture alignment

This keeps native socket types entirely behind the implementation boundary. No Winsock/POSIX types are added to the public API, and transport/DNS/HTTP semantics remain out of this layer.

Scope

This PR intentionally does not implement DNS resolution, socket creation policy, TCP connect, timeouts, send/receive loops, or Client behavior. Those build on this ownership primitive in follow-up PRs.