feat: add timed TCP read and write I/O

[zuudevs]

Summary

Completes the byte-oriented v1 TCP transport primitive by adding bounded synchronous reads and complete writes with partial-I/O handling.

Added

• src/platform/socket_mode.hpp / .cpp
  • reusable cross-platform blocking/non-blocking mode helper
  • shared by connect and I/O paths
• src/platform/socket_io.hpp / .cpp
  • readable/writable readiness waits
  • Windows select() / POSIX poll()
  • normalized send / recv attempts
  • native interrupted/would-block/connection-closed classification
  • MSG_NOSIGNAL where available
• TcpConnection::write_all(std::string_view, timeout)
  • loops until the entire byte sequence is sent
  • handles partial sends, EINTR, and would-block
  • one absolute timeout budget for the complete operation
  • returns bytes written on success
• TcpConnection::read_some(char*, capacity, timeout)
  • incremental byte read suitable for the future HTTP parser
  • handles EINTR, would-block, EOF, and timeout
  • returns bytes read on success
• macOS/BSD SO_NOSIGPIPE socket setup where MSG_NOSIGNAL is unavailable

Failure semantics

• peer EOF/reset maps to ConnectionClosed
• write timeout maps to WriteTimeout
• read timeout maps to ReadTimeout
• other send/receive failures map to WriteFailed / ReadFailed
• after an I/O timeout or unrecoverable I/O failure, the connection is closed conservatively so an incomplete HTTP byte stream cannot be reused accidentally
• empty writes and zero-capacity reads are successful no-ops

Tests

• complete 2 MiB loopback payload write with concurrent reader
• normal incremental read
• deterministic read timeout
• peer close detection
• deterministic zero-deadline write timeout
• zero-length read/write behavior

Tests use loopback sockets only and require no public network access.

CI cleanup

• removes the currently unused Google Benchmark vcpkg dependency; benchmarks/CMakeLists.txt is still empty, so CI no longer downloads/builds Benchmark unnecessarily

Scope

This PR completes the low-level synchronous TCP byte stream needed by the HTTP layer. It intentionally does not implement HTTP request serialization, response parsing, chunked decoding, redirects, or connection reuse policy yet.