Problem: Incrementing or decrementing priority required operating on one
task at a time with `<C-a>`/`<C-x>`, which is tedious when adjusting
multiple tasks.
Solution: Add `adjust_priority_visual(delta)` that iterates the visual
selection range, updates every task line's priority in one pass, then
re-renders once. Exposed as `increment_priority_visual()` /
`decrement_priority_visual()` with `g<C-a>` / `g<C-x>` defaults, new
`<Plug>` mappings, and config keys `priority_up_visual` /
`priority_down_visual`.
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