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  pack format, superseded by the ported 40250 code), their 18 unregistered tests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

.agents/skills/metin2-40250-parity-audit/references/project-map.md

@@ -65,7 +65,7 @@ The 106 `project/*_system.gd` files that only their own `test_*_parity.gd` loade
 
 ## Existing evidence (historical)
 
-- `docs/CLIENT-GAP.md`, `docs/CLIENT-PARITY-AUDIT-AND-FIX-GUIDE.md`, `docs/CLIENT-40250-PORT.md`,
+- `docs/CLIENT-GAP.md`, `docs/CLIENT-PARITY-AUDIT-AND-FIX-GUIDE.md`, `docs/archive/CLIENT-40250-PORT.md`,
   `docs/PARITY-GAP.md`: background only; trust the port-map, not their status text.
 - `oracle/run-diff-suite.sh`: Granny numerical comparison suite.