refactor: extract fallback textures, add why-comments, name WMO constant

- character_renderer: extract duplicated fallback texture creation
  (white/transparent/flat-normal) into createFallbackTextures() — was
  copy-pasted between initialize() and clear()
- wmo_renderer: replace magic 8192 with kMaxRetryTracked constant,
  add why-comment explaining the fallback-retry set cap (Dalaran has
  2000+ unique WMO groups)
- quest_handler: add why-comment on reqCount=0 fallback — escort/event
  quests can report kill credit without objective counts in query response

include/rendering/character_renderer.hpp

@@ -251,6 +251,10 @@ public:
 
 
 private:
+    // Create 1×1 fallback textures used when real textures are missing or still loading.
+    // Called during both init and clear to ensure valid descriptor bindings at all times.
+    void createFallbackTextures(VkDevice device);
+
     VkContext* vkCtx_ = nullptr;
     VkRenderPass renderPassOverride_ = VK_NULL_HANDLE;
     VkSampleCountFlagBits msaaSamplesOverride_ = VK_SAMPLE_COUNT_1_BIT;