New parts, lists and comments took "largest id + 1": after an id of
u32::MAX (a .json model or a .docx can carry one) that overflowed, a
panic in debug builds and a wrap to 0 in release, and list.define's
saturating_add reused the taken id. free_id gives one past the largest,
or the lowest free id once that would overflow; try_add_part,
try_add_list and try_add_levels report ids running out and add nothing,
and the commands that add parts and lists use them.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: Brandon Thomas <echelon@gmail.com>