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  pack format, superseded by the ported 40250 code), their 18 unregistered tests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/reference/README.md

@@ -2,7 +2,7 @@
 
 These docs come from **`xrender-poc`**, the bgfx-based rendering POC that was
 **retired on 2026-08-29** in favour of the Godot route (this repo). See
-[`../MIDREVIEW.md`](../MIDREVIEW.md) for the decision.
+[`../MIDREVIEW.md`](../archive/MIDREVIEW.md) for the decision.
 
 They are kept because the reverse-engineering knowledge is route-independent and
 still authoritative: