feat: add DNS resolver and endpoint candidates

[zuudevs]

Summary

Adds the v1 DNS/address-resolution layer that turns a host/port pair into owned IPv4/IPv6 TCP endpoint candidates.

Added

• src/net/endpoint.hpp
  • internal owned sockaddr_storage value type
  • preserves family, socket type, protocol, and native address length
  • IPv4/IPv6 helpers
  • copies native address data so candidates do not depend on addrinfo lifetime
• src/net/resolver.hpp / src/net/resolver.cpp
  • getaddrinfo-based resolution
  • AF_UNSPEC IPv4 + IPv6 discovery
  • TCP stream hints
  • preserves resolver ordering
  • frees native addrinfo through RAII
  • maps resolver/runtime failures to ErrorCode::ResolveFailed
• src/platform/network_runtime.hpp / .cpp
  • internal network runtime bootstrap
  • Windows WSAStartup(2.2) once through function-local static RAII
  • matching WSACleanup()
  • POSIX no-op implementation
• tests/resolver_test.cpp
  • endpoint address-copy ownership
  • numeric IPv4 loopback resolution
  • numeric IPv6 loopback resolution
  • port preservation
  • invalid/empty hostname errors

Build changes

• compile resolver and network runtime into the internal target
• wire resolver tests into the existing GoogleTest target

CI / determinism

Tests use numeric loopback addresses (127.0.0.1 and ::1) and do not depend on public DNS or internet connectivity.

Scope

This PR intentionally does not create sockets or attempt TCP connections. Candidate iteration, connect timeouts, partial I/O, and connection reuse belong to the upcoming TcpConnection transport work.