Function resolve_device 

pub fn resolve_device(spec: &str) -> Result<Device, DeviceError>

Resolve a CLI --device string into a concrete Device.

Contract: this function is THE single binding point for INV-GPUTRAIN-001 (grammar) and GATE-GPUTRAIN-002 (no silent CPU fallback on explicit CUDA request).

Errors

• DeviceError::InvalidSpec — spec is not one of cpu, cuda, cuda:N (0..=15), or auto.
• DeviceError::CudaNotAvailable — spec explicitly asked for CUDA (or auto chose CUDA) but cuda_training_available() returned false.