Verify kernel archive integrity (#1703)

Closes https://github.com/apple/container/issues/1687

The default kernel archive is downloaded from a remote release URL
during first-run setup and via `container system kernel set
--recommended`. Previously, the archive contents were not verified after
download, so integrity depended on HTTPS and the release artifact
remaining unchanged.

This change adds digest verification for kernel archives. The
recommended/default kernel now has pinned digest metadata using an
algorithm-prefixed value such as `sha256:<hex>`. `container system
kernel set --tar` accepts `--digest`; remote tar URLs require it, and
local tar archives can also be verified before unpacking and
installation.

The system config also supports `kernel.digest`, and a custom
`kernel.url` must provide a digest for that archive.

docs/command-reference.md

@@ -1543,7 +1543,7 @@ Installs or updates the Linux kernel used by the container runtime on macOS host
 **Usage**
 
 ```bash
-container system kernel set [--arch <arch>] [--binary <binary>] [--force] [--recommended] [--tar <tar>] [--debug]
+container system kernel set [--arch <arch>] [--binary <binary>] [--force] [--recommended] [--tar <tar>] [--digest <digest>] [--debug]
 ```
 
 **Options**
@@ -1553,6 +1553,7 @@ container system kernel set [--arch <arch>] [--binary <binary>] [--force] [--rec
 *   `--force`: Overwrites an existing kernel with the same name
 *   `--recommended`: Download and install the recommended kernel as the default (takes precedence over all other flags)
 *   `--tar <tar>`: Filesystem path or remote URL to a tar archive containing a kernel file
+*   `--digest <digest>`: Expected digest for the tar archive, for example `sha256:<hex>`. Required when `--tar` is a remote URL.
 
 ### `container system property list (ls)`