Fix WAV decode cache and spell lookup performance

- audio_engine.cpp: cache key was based on vector pointer address, not
  content — cache never hit since each asset load produces a new
  allocation. Replace with FNV-1a over head+tail bytes + size, giving
  correct content-based identity at O(1) cost. Add 256-entry cap with
  eviction to prevent unbounded growth over long sessions.

- game_handler.hpp/cpp, spellbook_screen, game_screen: knownSpells was
  a std::vector with O(n) std::find lookups scattered across packet
  handlers and UI code. Switch to std::unordered_set for O(1) insert,
  erase, and membership checks. Update all push_back/erase-remove/find
  call sites to use set operations.

include/ui/spellbook_screen.hpp

@@ -73,7 +73,7 @@ private:
     void loadSpellDBC(pipeline::AssetManager* assetManager);
     void loadSpellIconDBC(pipeline::AssetManager* assetManager);
     void loadSkillLineDBCs(pipeline::AssetManager* assetManager);
-    void categorizeSpells(const std::vector<uint32_t>& knownSpells);
+    void categorizeSpells(const std::unordered_set<uint32_t>& knownSpells);
     GLuint getSpellIcon(uint32_t iconId, pipeline::AssetManager* assetManager);
     const SpellInfo* getSpellInfo(uint32_t spellId) const;
 };