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.

src/ui/spellbook_screen.cpp

@@ -136,7 +136,7 @@ void SpellbookScreen::loadSkillLineDBCs(pipeline::AssetManager* assetManager) {
     }
 }
 
-void SpellbookScreen::categorizeSpells(const std::vector<uint32_t>& knownSpells) {
+void SpellbookScreen::categorizeSpells(const std::unordered_set<uint32_t>& knownSpells) {
     spellTabs.clear();
 
     // Only SkillLine category 7 ("Class") gets its own tab (the 3 specialties).