- Add --vulkan-device argument to select specific GPU (for multi-GPU systems)
- Add --vulkan-list-devices to list available Vulkan GPUs
- Update VulkanBackend to use main_gpu parameter for device selection
- Add list_vulkan_devices() method to show available devices
- Update README with new command-line options and examples

Useful when you have both NVIDIA and AMD GPUs and want to ensure
Vulkan uses the AMD GPU specifically.