Performance: ring buffer UBOs, batched load screen uploads, background world preloader

- Replace per-frame VMA alloc/free of material UBOs with a ring buffer in
  CharacterRenderer (~500 allocations/frame eliminated)
- Batch all ready terrain tiles into a single GPU upload during load screen
  (processAllReadyTiles instead of one-at-a-time with individual fence waits)
- Lift per-frame creature/GO spawn budgets during load screen warmup phase
- Add background world preloader: saves last world position to disk, pre-warms
  AssetManager file cache with ADT files starting at app init (login screen)
  so terrain workers get instant cache hits when Enter World is clicked
- Distance-filter expensive collision guard to 8-unit melee range
- Merge 3 CharacterRenderer update loops into single pass
- Time-budget instrumentation for slow update stages (>3ms threshold)
- Count-based async creature model upload budget (max 3/frame in-game)
- 1-per-frame game object spawn + per-doodad time budget for transport loading
- Use deque for creature spawn queue to avoid O(n) front-erase

src/game/game_handler.cpp

@@ -541,7 +541,13 @@ void GameHandler::update(float deltaTime) {
 
     // Update socket (processes incoming data and triggers callbacks)
     if (socket) {
+        auto socketStart = std::chrono::steady_clock::now();
         socket->update();
+        float socketMs = std::chrono::duration<float, std::milli>(
+            std::chrono::steady_clock::now() - socketStart).count();
+        if (socketMs > 3.0f) {
+            LOG_WARNING("SLOW socket->update: ", socketMs, "ms");
+        }
     }
 
     // Detect server-side disconnect (socket closed during update)