Fold a ledger subject

Requires a workspace-bound API key with ledger:read. Returns the fold of one subject: the head of every claim key that touches it. recordedAt selects what the ledger had recorded by then; validAt keeps only heads whose valid interval contains that time. A designation outside validAt moves to unlinkedDesignations instead of disappearing. Retracted heads are listed separately. A pair with both a live same-as and a live distinct-from is contested; the fold picks no winner and never merges rows. Reads are bounded; truncated is true when a budget was reached.