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

docs/srp-implementation.md

@@ -351,13 +351,13 @@ The expensive operation (session key computation) only happens once per login.
 2. **No Plaintext Storage:** Password is immediately hashed, never stored
 3. **Forward Secrecy:** Ephemeral keys (a, A) are generated per session
 4. **Mutual Authentication:** Both client and server prove knowledge of password
-5. **Secure Channel:** Session key K can be used for encryption (not implemented yet)
+5. **Secure Channel:** Session key K is used for RC4 header encryption after auth completes
 
 ## References
 
 - [SRP Protocol](http://srp.stanford.edu/)
 - [WoWDev Wiki - SRP](https://wowdev.wiki/SRP)
-- Original wowee: `/wowee/src/lib/crypto/srp.js`
+- Implementation: `src/auth/srp.cpp`, `include/auth/srp.hpp`
 - OpenSSL BIGNUM: https://www.openssl.org/docs/man1.1.1/man3/BN_new.html
 
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