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  pack format, superseded by the ported 40250 code), their 18 unregistered tests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/tests/port_packet_test.cpp

@@ -1,4 +1,4 @@
-// 批次 2V1: the ported 40250 UserInterface/Packet.h against the classic wire layouts in net/classic/wire_classic.h,
+// 批次 2V1: the ported 40250 UserInterface/Packet.h against the classic wire layouts in wire_classic.h,
 // which the mtnet backend checked 1:1 against the 40250 server on 192.168.21.203. Win32 `long` is 32 bits and
 // Packet.h now says LONG; a 64-bit long, or a missing #pragma pack(1), shows up here as a size mismatch.
 // Covers the packets the login/select/loading phases send and read. The two headers cannot share a translation unit