fix(buffer): correct extmark drift on open_line above/below done tasks

Problem: `open_line` used `nvim_buf_set_lines` which triggered `on_bytes`
with a `start_row` offset designed for native `o`/`O` keypresses. The
`_meta` entry was inserted one position too late, causing the done task's
`PendingDone` highlight to attach to the new blank line instead.

Solution: suppress `on_bytes` during `open_line` by reusing the
`_rendering` guard, insert the meta entry at the correct position, and
immediately reapply inline extmarks for the affected rows.

lua/pending/buffer.lua

@@ -345,8 +345,25 @@ function M.open_line(above)
   end
   local row = vim.api.nvim_win_get_cursor(0)[1]
   local insert_row = above and (row - 1) or row
+  local meta_pos = insert_row + 1
+
+  _rendering = true
   vim.bo[bufnr].modifiable = true
   vim.api.nvim_buf_set_lines(bufnr, insert_row, insert_row, false, { '- [ ] ' })
+  _rendering = false
+
+  table.insert(_meta, meta_pos, { type = 'task' })
+
+  local icons = config.get().icons
+  local total = vim.api.nvim_buf_line_count(bufnr)
+  for r = meta_pos, math.min(meta_pos + 1, total) do
+    vim.api.nvim_buf_clear_namespace(bufnr, ns_inline, r - 1, r)
+    local m = _meta[r]
+    if m then
+      apply_inline_row(bufnr, r - 1, m, icons)
+    end
+  end
+
   vim.api.nvim_win_set_cursor(0, { insert_row + 1, 6 })
   vim.cmd('startinsert!')
 end