docs: fix stale references across 10 documentation files

- CONTRIBUTING.md: C++17 → C++20 (matches CMakeLists.txt)
- TROUBLESHOOTING.md: fix log path (~/.wowee/logs/ → logs/wowee.log)
- docs/authentication.md: remove stale "next milestone" (char enum
  and world entry have been working for months)
- docs/srp-implementation.md: update session key status (RC4 encryption
  is implemented), fix file reference to actual src/auth/srp.cpp
- docs/packet-framing.md: remove stale "next steps" (realm list is
  fully implemented), update status with tested servers
- docs/WARDEN_IMPLEMENTATION.md: fix file list — handler is in
  warden_handler.cpp not game_handler.cpp, add warden_memory.hpp/cpp
- docs/WARDEN_QUICK_REFERENCE.md: fix header/source paths (include/
  not src/), add warden_handler and warden_memory
- docs/quickstart.md: fix clone command (--recurse-submodules, WoWee
  not wowee), remove obsolete manual ImGui clone step, fix log path
- docs/server-setup.md: update version to v1.8.9-preview, date to
  2026-03-30, add all supported expansions
- assets/textures/README.md: remove broken doc references
  (TURTLEHD_IMPORT.md, TEXTURE_MANIFEST.txt), update integration
  status to reflect working PNG override pipeline

CONTRIBUTING.md

@@ -12,7 +12,7 @@ make -C build -j$(nproc)
 
 ## Code Style
 
-- **C++17**. Use `#pragma once` for include guards.
+- **C++20**. Use `#pragma once` for include guards.
 - Namespaces: `wowee::game`, `wowee::rendering`, `wowee::ui`, `wowee::core`, `wowee::network`.
 - Conventional commit messages in imperative mood:
   - `feat:` new feature