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/game_screen.cpp

@@ -4860,7 +4860,7 @@ void GameScreen::renderTrainerWindow(game::GameHandler& gameHandler) {
             auto isKnown = [&](uint32_t id) {
                 if (id == 0) return true;
                 // Check if spell is in knownSpells list
-                bool found = std::find(knownSpells.begin(), knownSpells.end(), id) != knownSpells.end();
+                bool found = knownSpells.count(id);
                 if (found) return true;
 
                 // Also check if spell is in trainer list with state=2 (explicitly known)