Add crates/weft-appd/src/mount.rs with MountOrchestrator.
On each app launch, supervise() calls MountOrchestrator::mount_if_needed
which looks for <app_id>.app.img, <app_id>.hash, and <app_id>.roothash
in the app store roots. If all three exist and weft-mount-helper is
available (WEFT_MOUNT_HELPER env or /usr/lib/weft/ default paths), it:
- creates /tmp/weft-mnt-<session_id>/<app_id>/
- invokes weft-mount-helper mount to set up dm-verity and EROFS mount
- sets WEFT_APP_STORE=/tmp/weft-mnt-<session_id> in the child env so
the runtime resolves the package from the mounted read-only image
After the process exits, umount() invokes weft-mount-helper umount and
removes the temporary directory.
Falls back to directory-based install silently if no image is found,
mount-helper is absent, or the mount fails.
Test: find_image_returns_none_when_absent.
