Function resolve_kernel_image 

pub fn resolve_kernel_image(
    kernel: Option<&str>,
    policy: &KernelResolvePolicy<'_>,
) -> Result<PathBuf>

Resolve a kernel identifier to a bootable image path.

Handles KernelId variants: directory (auto-build), version string, and cache key. Raw image file acceptance is controlled by policy.accept_raw_image. The None case resolves automatically via cache then filesystem, falling back to auto-download.