DXN duration (@duration "P..."). Each field is
integer() | nil — only fields actually present in the source set
a bit in .dxnb's bitmask (DXN.md §2.3). years/months are
calendar-relative (variable length) and not reducible to a single
elapsed-time scalar, so all seven fields are tracked independently
rather than folded into one (DXN.md §1.4).