Diff a session's HardwareManifest against what is present, returning the absent
devices with what the restored show depends on each for (issue #154). Never blocks restore:
the caller only surfaces this list. An empty result means every expected device is present.
A session with no manifest (older save) has nothing to diff → an empty list.
Diff a session's HardwareManifest against what is present, returning the absent devices with what the restored show depends on each for (issue #154). Never blocks restore: the caller only surfaces this list. An empty result means every expected device is present. A session with no manifest (older save) has nothing to diff → an empty list.