Problem: Deleting lines (`dd`, `dat`, `d3j`) left extmarks stranded on
adjacent rows since `render()` only clears and reapplies marks on `:w`.
Quickfix `<CR>` opened the pending buffer in a second window because
`BufEnter` did not redirect to `task_winid`. Category fold state was
lost across `<Tab>/<Tab>` view toggles because `render()` overwrote the
saved state with an empty snapshot taken while folds were disabled.
Solution: Add a `TextChanged`/`TextChangedI` autocmd that clears the
extmark namespace immediately on any edit. Fix `BufEnter` to close
duplicate windows and redirect focus to `task_winid`, updating it when
stale. Fix `snapshot_folds` to skip if a state is already saved, and
`restore_folds` to always clear the saved state; snapshot in
`toggle_view` before the view flips so the state survives the round-trip.
559ab863a8