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/rendering/terrain_manager.cpp

@@ -1082,7 +1082,7 @@ void TerrainManager::workerLoop() {
 void TerrainManager::processReadyTiles() {
     // Process tiles with time budget to avoid frame spikes
     // Taxi mode gets a slightly larger budget to avoid visible late-pop terrain/models.
-    const float timeBudgetMs = taxiStreamingMode_ ? 8.0f : 5.0f;
+    const float timeBudgetMs = taxiStreamingMode_ ? 8.0f : 3.0f;
     auto startTime = std::chrono::high_resolution_clock::now();
 
     // Move newly ready tiles into the finalizing deque.