fix: VkTexture move/destroy ownsSampler_ flag, extract finalizeSampler

- Fix move constructor and move assignment: set other.ownsSampler_ to
  false after transfer (was incorrectly set to true, leaving moved-from
  object claiming ownership of a null sampler)
- Fix destroy(): reset ownsSampler_ to false after clearing sampler
  handle (was set to true, inconsistent with null handle state)
- Extract finalizeSampler() from 3 duplicated cache-or-create blocks
  in createSampler() overloads and createShadowSampler() (-24 lines)
- Add SPIR-V alignment why-comment in vk_shader.cpp

src/rendering/vk_shader.cpp

@@ -32,6 +32,7 @@ bool VkShaderModule::loadFromFile(VkDevice device, const std::string& path) {
     }
 
     size_t fileSize = static_cast<size_t>(file.tellg());
+    // SPIR-V is a stream of 32-bit words — file size must be a multiple of 4
     if (fileSize == 0 || fileSize % 4 != 0) {
         LOG_ERROR("Invalid SPIR-V file size (", fileSize, "): ", path);
         return false;