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  pack format, superseded by the ported 40250 code), their 18 unregistered tests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>

extension/third_party/CMakeLists.txt

@@ -1,12 +1,10 @@
 # Vendored native dependencies for the 40250 port and SDL/Vulkan client.
 #
-# Phase 1 (macOS) used Homebrew for libsodium / libzstd / liblzo2. Those don't
-# exist on the Android NDK or iOS SDK sysroots, so the mobile bring-up (BACKLOG
-# F1/F2) needs them built from source as part of our own build. All three are
-# pinned here and produce static libraries for port_platform:
+# Everything is built from source as static libraries (the Android NDK and iOS
+# SDK sysroots have none of them):
 #
-#   sodium          <- libsodium-cmake submodule (wraps jedisct1/libsodium)
-#   libzstd_static  <- facebook/zstd submodule, its own build/cmake project
+#   cryptopp        <- Crypto++ (classic protocol cipher)
+#   freetype        <- FreeType 2.13.3 (platform font adapter)
 #   minilzo         <- vendored miniLZO source (LZO1X, ~4 files) — see NOTICE
 #
 # See docs/THIRD-PARTY.md for versions / licenses. liblzo2 (and thus miniLZO) is
@@ -16,24 +14,6 @@
 # Android links these static libraries into libmain.so, so they must be PIC.
 set(CMAKE_POSITION_INDEPENDENT_CODE ON)
 
-# --- libsodium ------------------------------------------------------------------
-set(SODIUM_DISABLE_TESTS ON  CACHE BOOL "" FORCE)
-set(SODIUM_MINIMAL       OFF CACHE BOOL "" FORCE)
-add_subdirectory(libsodium-cmake EXCLUDE_FROM_ALL)
-# The wrapper's target is `sodium`; give it a namespaced alias for consumers.
-add_library(mt3p::sodium ALIAS sodium)
-
-# --- libzstd ------------------------------------------------------------------
-set(ZSTD_BUILD_PROGRAMS      OFF CACHE BOOL "" FORCE)
-set(ZSTD_BUILD_SHARED        OFF CACHE BOOL "" FORCE)
-set(ZSTD_BUILD_STATIC        ON  CACHE BOOL "" FORCE)
-set(ZSTD_BUILD_TESTS         OFF CACHE BOOL "" FORCE)
-set(ZSTD_BUILD_CONTRIB       OFF CACHE BOOL "" FORCE)
-set(ZSTD_LEGACY_SUPPORT      OFF CACHE BOOL "" FORCE)
-set(ZSTD_MULTITHREAD_SUPPORT OFF CACHE BOOL "" FORCE)
-add_subdirectory(zstd/build/cmake zstd-build EXCLUDE_FROM_ALL)
-add_library(mt3p::zstd ALIAS libzstd_static)
-
 # --- Crypto++ (MT_PROTOCOL=classic cipher) ----------------------------------
 add_subdirectory(cryptopp EXCLUDE_FROM_ALL)