Background normal map generation, queue-draining load screen warmup

- Normal map CPU work (luminance→blur→Sobel) moved to background threads,
  main thread only does GPU upload (~1-2ms vs 15-22ms per texture)
- Load screen warmup now waits until ALL spawn/equipment/gameobject queues
  are drained before transitioning (prevents naked character, NPC pop-in)
- Exit condition: min 2s + 5 consecutive empty iterations, hard cap 15s
- Equipment queue processes 8 items per warmup iteration instead of 1
- Added LoadingScreen::renderOverlay() for future world-behind-loading use

include/rendering/character_renderer.hpp

@@ -13,6 +13,8 @@
 #include <utility>
 #include <future>
 #include <deque>
+#include <mutex>
+#include <atomic>
 
 namespace wowee {
 namespace pipeline { class AssetManager; }
@@ -304,15 +306,23 @@ private:
     std::unique_ptr<VkTexture> generateNormalHeightMap(
         const uint8_t* pixels, uint32_t width, uint32_t height, float& outVariance);
 
-    // Deferred normal map generation — avoids stalling loadModel
-    struct PendingNormalMap {
+    // Background normal map generation — CPU work on thread pool, GPU upload on main thread
+    struct NormalMapResult {
         std::string cacheKey;
-        std::vector<uint8_t> pixels;  // RGBA pixel data
+        std::vector<uint8_t> pixels;  // RGBA normal map output
         uint32_t width, height;
+        float variance;
     };
-    std::deque<PendingNormalMap> pendingNormalMaps_;
+    // Completed results ready for GPU upload (populated by background threads)
+    std::mutex normalMapResultsMutex_;
+    std::deque<NormalMapResult> completedNormalMaps_;
+    std::atomic<int> pendingNormalMapCount_{0};  // in-flight background tasks
+
+    // Pure CPU normal map generation (thread-safe, no GPU access)
+    static NormalMapResult generateNormalHeightMapCPU(
+        std::string cacheKey, std::vector<uint8_t> pixels, uint32_t width, uint32_t height);
 public:
-    void processPendingNormalMaps(int budget = 2);
+    void processPendingNormalMaps(int budget = 4);
 private:
 
     // Normal mapping / POM settings

include/rendering/loading_screen.hpp

@@ -24,6 +24,10 @@ public:
     // Render the loading screen with progress bar and status text (pure ImGui)
     void render();
 
+    // Draw loading screen as ImGui overlay (call within an existing ImGui frame).
+    // Used during warmup to overlay loading screen on top of the rendered world.
+    void renderOverlay();
+
     void setProgress(float progress) { loadProgress = progress; }
     void setStatus(const std::string& status) { statusText = status; }