GPU-01 Vulkan context in _ext #9

Goal

Process-wide Vulkan init: load loader, create instance/device/compute queue, fail cleanly if unavailable.

Todos

• Add CMake option CTHREADS_GPU (off by default).
• Dynamically load vulkan-1 / libvulkan.so.1 (no hard link required for CPU builds).
• Implement gpu_init() / gpu_shutdown() (lazy or explicit).
• Pick first compute-capable device (prefer discrete).
• Expose minimal Python: cthreads.gpu.available() -> bool, cthreads.gpu.device_name() -> str.
• Raise typed errors: loader missing / no device / init failed.

Notes

• Interface: no buffers yet; probe-only API.
• Arch: singleton in _ext; thread-safe call_once init.
• Impl: Vulkan 1.1+ if easy; don’t require validation layers unless CTHREADS_VK_VALIDATE=1.
• User dep: GPU drivers only - never Vulkan SDK.