Interface StableIdInventory

The state of one tree: its identity-bearing elements plus its lesson files. retired carries the tree's declared retirements (each set manifest's metadata.retired_ids, engine#131); absent means "none declared" and keeps pre-existing callers valid.

interface StableIdInventory {
    elements: StableIdElement[];
    lessons: { filename: string; set: string }[];
    retired?: { set: string; stableId: string }[];
}