Harden Vulkan init/device selection and resource teardown

- Remove unconditional VK_KHR_swapchain request; offscreen renderer needs
  no device extensions, fixing failures on headless/CI setups.
- Enumerate physical devices properly and pick the first one with a
  graphics queue family instead of blindly choosing device 0.
- Check vkEnumeratePhysicalDevices/vkBindImageMemory/vkBindBufferMemory
  results.
- Validate FindMemoryType() result before allocation and log clear errors.
- Add a persistent linear sampler created in PostInit and destroy it in
  DestroyInternal.
- Fix DestroyInternal() early-return leak: now tears down the instance
  even if device creation failed.

ctest passes 4/4.

app/render/vulkan/vulkanrenderer.h

@@ -129,6 +129,7 @@ private:
 
 	VkInstance instance_ = VK_NULL_HANDLE;
 	VkPhysicalDevice physical_device_ = VK_NULL_HANDLE;
+	uint32_t physical_device_count_ = 0;
 	VkDevice device_ = VK_NULL_HANDLE;
 	VkQueue graphics_queue_ = VK_NULL_HANDLE;
 	uint32_t graphics_queue_family_ = UINT32_MAX;