Problem: priority was binary (0 or 1), toggled with !, with no way
to express finer gradations or use Vim's native increment idiom.
Solution: replace toggle_priority with change_priority(delta) which
clamps to floor 0. Display format changes from '! ' to '[N] ' so any
integer level is representable. Parser updated to extract numeric
level from the [N] prefix. Visual g<C-a>/g<C-x> apply the delta to
all tasks in the selection. <Plug>(pending-priority) replaced with
<Plug>(pending-priority-up) and <Plug>(pending-priority-down).
