Fixes #3560

This PR fixes a startup crash that occurs on systems without a Vulkan-capable GPU.

Previously, Graphite would select a CPU-based Vulkan adapter (such as llvmpipe) and continue initialization. This would later trigger a wgpu validation error, followed by a panic and segmentation fault, making it difficult for users to understand that their system does not meet the graphics requirements.

Summary of changes

• Detect and reject CPU-based Vulkan adapters during context initialization.

• Fail early when no supported GPU adapter is available, instead of continuing into an invalid wgpu state.

• Preserve existing adapter-selection logic while adding a clear support check to prevent crashes.

Impact

• Prevents crashes and undefined behavior on unsupported systems.

• Makes Graphite’s GPU requirements clearer to users.

• Improves robustness and error handling during graphics initialization.

Testing

• Confirmed normal startup on systems with a supported GPU.

• Confirmed graceful failure (no panic or segmentation fault) when only CPU Vulkan adapters are present.

• Verified with cargo check and cargo clippy --all-targets --all-features.