fix(buffer): fix stale extmarks, duplicate window, and fold state loss (#92)

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.

lua/pending/buffer.lua

@@ -77,6 +77,18 @@ function M.clear_winid()
   task_winid = nil
 end
 
+---@param winid integer
+---@return nil
+function M.update_winid(winid)
+  task_winid = winid
+end
+
+---@param b? integer
+---@return nil
+function M.clear_marks(b)
+  vim.api.nvim_buf_clear_namespace(b or task_bufnr, task_ns, 0, -1)
+end
+
 ---@return nil
 function M.close()
   if not task_winid or not vim.api.nvim_win_is_valid(task_winid) then
@@ -263,6 +275,9 @@ local function snapshot_folds(bufnr)
     return
   end
   for _, winid in ipairs(vim.fn.win_findbuf(bufnr)) do
+    if _fold_state[winid] ~= nil then
+      goto continue
+    end
     local state = {}
     vim.api.nvim_win_call(winid, function()
       for lnum, m in ipairs(_meta) do
@@ -274,6 +289,7 @@ local function snapshot_folds(bufnr)
       end
     end)
     _fold_state[winid] = state
+    ::continue::
   end
 end
 
@@ -283,6 +299,7 @@ local function restore_folds(bufnr)
   end
   for _, winid in ipairs(vim.fn.win_findbuf(bufnr)) do
     local state = _fold_state[winid]
+    _fold_state[winid] = nil
     if state and next(state) ~= nil then
       vim.api.nvim_win_call(winid, function()
         vim.cmd('normal! zx')
@@ -295,7 +312,6 @@ local function restore_folds(bufnr)
         end
         vim.api.nvim_win_set_cursor(0, saved)
       end)
-      _fold_state[winid] = nil
     end
   end
 end
@@ -372,6 +388,7 @@ end
 
 ---@return nil
 function M.toggle_view()
+  snapshot_folds(task_bufnr)
   if current_view == 'category' then
     current_view = 'priority'
   else