feat(core): port vulkan_loader_available from src-tauri

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>

crates/core/Cargo.toml

@@ -12,6 +12,7 @@ sysinfo = "0.35"
 async-trait = "0.1"
 num_cpus = "1"
 tracing = "0.1"
+libloading = "0.8"
 
 [dev-dependencies]
 tempfile = "3"