Batch GPU uploads to eliminate per-upload fence waits (stutter fix)

Every uploadBuffer/VkTexture::upload called immediateSubmit which did a
separate vkQueueSubmit + vkWaitForFences. Loading a single creature model
with textures caused 4-8+ fence waits; terrain chunks caused 80+ per batch.

Added beginUploadBatch/endUploadBatch to VkContext: records all upload
commands into a single command buffer, submits once with one fence wait.
Staging buffers are deferred for cleanup after the batch completes.

Wrapped in batch mode:
- CharacterRenderer::loadModel (creature VB/IB + textures)
- M2Renderer::loadModel (doodad VB/IB + textures)
- TerrainRenderer::loadTerrain/loadTerrainIncremental (chunk geometry + textures)
- TerrainRenderer::uploadPreloadedTextures
- WMORenderer::loadModel (group geometry + textures)

include/rendering/vk_context.hpp

@@ -1,5 +1,6 @@
 #pragma once
 
+#include "rendering/vk_utils.hpp"
 #include <vulkan/vulkan.h>
 #include <vk_mem_alloc.h>
 #include <VkBootstrap.h>
@@ -46,6 +47,13 @@ public:
     // Immediate submit for one-off GPU work (descriptor pool creation, etc.)
     void immediateSubmit(std::function<void(VkCommandBuffer cmd)>&& function);
 
+    // Batch upload mode: records multiple upload commands into a single
+    // command buffer, then submits with ONE fence wait instead of one per upload.
+    void beginUploadBatch();
+    void endUploadBatch();
+    bool isInUploadBatch() const { return inUploadBatch_; }
+    void deferStagingCleanup(AllocatedBuffer staging);
+
     // Accessors
     VkInstance getInstance() const { return instance; }
     VkPhysicalDevice getPhysicalDevice() const { return physicalDevice; }
@@ -143,6 +151,12 @@ private:
     VkCommandPool immCommandPool = VK_NULL_HANDLE;
     VkFence immFence = VK_NULL_HANDLE;
 
+    // Batch upload state (nesting-safe via depth counter)
+    int uploadBatchDepth_ = 0;
+    bool inUploadBatch_ = false;
+    VkCommandBuffer batchCmd_ = VK_NULL_HANDLE;
+    std::vector<AllocatedBuffer> batchStagingBuffers_;
+
     // Depth buffer (shared across all framebuffers)
     VkImage depthImage = VK_NULL_HANDLE;
     VkImageView depthImageView = VK_NULL_HANDLE;