Adds libloading = "0.8" to magnotia-core dependencies and moves
vulkan_loader_available() into magnotia-core::hardware so non-Tauri
crates (transcription, llm) can probe the Vulkan loader without
depending on the Tauri binary. Test asserts the call completes on any
host regardless of whether libvulkan is installed.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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