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  pack format, superseded by the ported 40250 code), their 18 unregistered tests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

docs/PARITY-GAP.md

@@ -1,7 +1,7 @@
 # 渲染一致性差距清单 —— 距 `ingame-shinsoo.png` 还差什么
 
-> 更新：2026-08-29 · 配套 [`SHINSOO-WORLD-RENDERING.md`](./SHINSOO-WORLD-RENDERING.md)（实施规格）、
-> [`BACKLOG.md`](./BACKLOG.md)。
+> 更新：2026-08-29 · 配套 [`SHINSOO-WORLD-RENDERING.md`](./archive/SHINSOO-WORLD-RENDERING.md)（实施规格）、
+> [`BACKLOG.md`](./archive/BACKLOG.md)。
 >
 > **本文回答一个问题**：现在 `test/golden/world-a1-{persp,r2}.png` 到「与游戏内截图目视等价」之间，
 > 还有哪些工作。不是 SHINSOO 的重复 —— SHINSOO 是「怎么把世界渲染出来」，本文是「怎么把观感做到