Added a GPU memory check using `getSystemInfo` to ensure Vulkan is selected only on systems with at least 6 GB of VRAM.
* Made `determineBestBackend` asynchronous and updated all callers to `await` it.
* Adjusted backend priority list to include or demote Vulkan based on the memory check.
* Updated Vulkan support detection in `backend.ts` to rely solely on API version (memory check moved to selection logic).
* Imported `getSystemInfo` and refined file‑existence validation.
These changes prevent sub‑optimal Vulkan usage on low‑memory GPUs and improve backend selection reliability.
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