2V1-a: port the EterLib network layer and the Crypto++ packet cipher

NetStream/NetAddress/NetDevice/NetPacketHeaderMap and EterBase/cipher.cpp
are mechanical copies. The winsock shim maps Winsock onto BSD sockets and
keeps the Winsock semantics CNetworkStream relies on: select ignores nfds,
a socket whose connect failed is not writable (40250 times out instead),
EINPROGRESS reads as WSAEWOULDBLOCK, and SIGPIPE is ignored.

port.net covers loopback connect, send/recv, peer close, the refused
connect timeout and the client/server key agreement.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

extension/src/port/CMakeLists.txt

@@ -174,6 +174,10 @@ if(BUILD_TESTING AND CMAKE_SYSTEM_NAME STREQUAL CMAKE_HOST_SYSTEM_NAME)
     add_test(NAME port.text COMMAND $<TARGET_FILE:port_text_test>)
     set_tests_properties(port.text PROPERTIES SKIP_RETURN_CODE 77)
 
+    add_executable(port_net_test ${CMAKE_CURRENT_SOURCE_DIR}/../../tests/port_net_test.cpp)
+    target_link_libraries(port_net_test PRIVATE port_platform)
+    add_test(NAME port.net COMMAND $<TARGET_FILE:port_net_test>)
+
     if(TARGET mtpython)
         add_executable(port_python_launcher_test ${CMAKE_CURRENT_SOURCE_DIR}/../../tests/port_python_launcher_test.cpp)
         target_link_libraries(port_python_launcher_test PRIVATE port_platform)