loadSessionStateSnapshotIntoStore

Apply a session snapshot to a store.

Defensive throughout, because the two halves can disagree: the snapshot was taken against a document that has since been edited by somebody else, or loaded next to a different document entirely. A page that is gone is skipped, a selection of deleted shapes is dropped, and a snapshot from a schema version this build does not know is ignored rather than half-applied — half-applied session state is how a person ends up on a blank page with a camera pointing at nothing.