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

assets/textures/README.md

@@ -0,0 +1,33 @@
+# HD Texture Assets
+
+**Source**: TurtleHD Texture Pack (Turtle WoW)
+**Imported**: 2026-01-27
+**Total Files**: 298 BLP textures
+**Total Size**: 10MB
+
+## Directory Structure
+
+```
+textures/
+├── character/
+│   └── human/          # 274 human male textures
+├── creature/           # 15 creature textures
+├── item/               # (reserved for future)
+└── world/
+    ├── generic/        # 1 generic world texture
+    └── stormwind/      # 8 Stormwind building textures
+```
+
+## Usage
+
+These HD BLP textures are ready for integration with:
+- **WMO Renderer**: Building texture mapping
+- **Character Renderer**: M2 model skin/face textures
+- **Creature Renderer**: NPC texture application
+
+## Integration Status
+
+Textures are loaded via the BLP pipeline and applied to WMO/M2 renderers.
+HD texture overrides (e.g. TurtleHD packs) can be placed as PNG files
+alongside the original BLP paths — the asset manager checks for `.png`
+overrides before loading the `.blp` version.