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)

src/rendering/character_renderer.cpp

@@ -1247,6 +1247,10 @@ bool CharacterRenderer::loadModel(const pipeline::M2Model& model, uint32_t id) {
     M2ModelGPU gpuModel;
     gpuModel.data = model;
 
+    // Batch all GPU uploads (VB, IB, textures) into a single command buffer
+    // submission with one fence wait, instead of one fence wait per upload.
+    vkCtx_->beginUploadBatch();
+
     // Setup GPU buffers
     setupModelBuffers(gpuModel);
 
@@ -1259,6 +1263,8 @@ bool CharacterRenderer::loadModel(const pipeline::M2Model& model, uint32_t id) {
         gpuModel.textureIds.push_back(texPtr);
     }
 
+    vkCtx_->endUploadBatch();
+
     models[id] = std::move(gpuModel);
 
     core::Logger::getInstance().debug("Loaded M2 model ", id, " (", model.vertices.size(),