- Add Windows make_local_stream_pair() via temp-listener emulation of
  socketpair(), enabling socket-pair tests on IOCP
- Implement assign_socket() in win_local_stream_service so raw SOCKET
  handles can be registered with the IOCP port
- Replace ConnectEx/AcceptEx with blocking connect()/accept() on
  worker threads for AF_UNIX — the IOCP extension functions are not
  reliable for AF_UNIX on all Windows versions
- Add portable temp_socket_dir helper using std::filesystem for
  temp paths across platforms
- Guard local datagram code (SOCK_DGRAM) as POSIX-only at compile time
  — Windows does not support AF_UNIX SOCK_DGRAM
- Remove dead IOCP datagram implementation files
  (win_local_dgram_service.hpp, win_local_dgram_socket.hpp)
- Document Windows limitation on local_datagram_socket and
  local_datagram headers

Adds targeted unit tests across the public API and native shadow layers
to close the largest line-coverage gaps in the develop baseline.

Major areas:

- tls_context: cover malformed PEM, mismatched cert/key, cipher list,
  protocol version round-trip, verify mode/depth, hostname, SNI/ALPN,
  CRL, OCSP staple, password callback (24% to 100%).
- io_context and scheduler detail: backend tag construction, run_for,
  run_until, restart, post-throws, deterministic multithreaded
  notify_one + wait_for coverage.
- socket_option: set/get round-trip for every public option plus the
  native templated variants; wrong-protocol error paths.
- local sockets: connect/accept error paths, abstract namespace,
  path-length boundary, mid-flight cancel, datagram send/recv. Adopt
  the temp_socket_dir helper introduced upstream.
- reactor internals: concurrent read/write on same descriptor,
  mid-flight cancel, close-during-op, wait_type::error paths, stop-token
  cancellation. Add reactor_paths.cpp aggregating these scenarios.
- tcp_server, posix_resolver_service, host_name, file services:
  lifecycle, accept loop, flag variants, error mapping.
- Add testConstructionWithThreadPoolSize regression for the resolver
  use_service fix.

Cross-platform reconciliation observed in CI:
- BSD-family kernels reject multicast set_option values (zero buffer
  sizes, leave-without-route, IPV6 ifindex 0) that Linux accepts;
  wrap in try/catch with documented platform variation.
- macOS returns EMSGSIZE for zero-length UDP datagrams; broaden the
  expectation to any error so MinGW and others pass too.
- Windows IOCP: gate testIoContextOptionsMaxEventsZero/BudgetInitClamp
  and the single-threaded resolver tests as POSIX-only; relax buffer
  size assertions to permit Windows's accept-zero-as-zero contract.
- POSIX-guard local socket tests that exercise abstract namespace and
  related Linux-only behavior.
- testMultithreadedNotifyAndWaitFor restructured to depend only on a
  work guard and counter drain, not on wall-clock timing, so it
  remains deterministic under thread-sanitizer.
- Signal-set shutdown test switched from POSIX-only SIGUSR1/2 to the
  portable SIGINT/SIGTERM pair.

Mirror asio::local::connect_pair as a free function in
boost::corosio. POSIX uses socketpair(); Windows performs a
private bind/listen/accept on the caller thread paired with a
connect on a short-lived worker thread, so the caller's
io_context is never driven.

Returns std::error_code (noexcept). Stream and POSIX-only
datagram overloads. native_local_stream_socket<Backend> slices
to the base parameter; assign() routes through the backend
service.

Replaces the test-only make_local_*_pair helpers from 4b952ec;
tests, benchmarks, and the user guide are migrated.