cleanup: drop Godot-era build trees, m2dev net/pack layers and stale docs

- Untrack build-debug/ and build-asan/ (CMake trees committed by mistake in
  c9389431) and ignore them.
- Remove extension/src/net and extension/src/pack (m2dev protocol and XChaCha
  pack format, superseded by the ported 40250 code), their 18 unregistered tests
  and the net/pack tools, the MT_BUILD_NET_TOOLS probe, and the libsodium and zstd
  submodules they alone used.
- The fake login server's classic cipher, sequence table and wire layouts move to
  extension/tests/classic/ (still used by port.login_flow, port.packet and the
  native client's offline mode).
- Archive Godot/m2dev-era plans under docs/archive/; THIRD-PARTY.md updated.

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

CMakeLists.txt

@@ -13,7 +13,6 @@ if(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
 endif()
 
 option(MT_BUILD_TOOLS "Build libgr2 validation tools" OFF)
-option(MT_BUILD_NET_TOOLS "Build the host-only classic protocol probe" OFF)
 option(MT_BUILD_PORT "Build the 40250 port and its tests" ON)
 option(MT_BUILD_NATIVE_RENDER "Build the SDL3 and Vulkan client" OFF)
 
@@ -29,27 +28,6 @@ if(MT_BUILD_TOOLS)
     add_subdirectory(tools)
 endif()
 
-# The protocol probe remains useful on the archived server's FreeBSD host.
-if(MT_BUILD_NET_TOOLS)
-    add_library(mtnet_host STATIC
-        extension/src/net/text_codec.cpp
-        extension/src/net/secure_cipher.cpp
-        extension/src/net/net_stream.cpp
-        extension/src/net/entity_store.cpp
-        extension/src/net/mark_image.cpp
-        extension/src/net/classic/classic_stream.cpp
-        extension/src/net/classic/classic_parser.cpp
-        extension/src/net/classic/classic_session.cpp
-        extension/src/net/classic/classic_cipher.cpp
-    )
-    target_include_directories(mtnet_host PUBLIC extension/src/net)
-    target_link_libraries(mtnet_host PUBLIC mt3p::sodium mt3p::minilzo mt3p::cryptopp mt_codepage)
-    target_compile_features(mtnet_host PUBLIC cxx_std_20)
-
-    add_executable(net_classic_e2e_host extension/tools/net_classic_e2e.cpp)
-    target_link_libraries(net_classic_e2e_host PRIVATE mtnet_host)
-endif()
-
 if(MT_BUILD_NATIVE_RENDER)
     add_subdirectory(native_render)
 endif()