fix(insert): pumvisible() guard on smart_return/smart_shift_return (audit follow-up)

The mappings re-audit confirmed the two new bindings (visual <CR> normalize,
insert <S-CR> multiline item) are correct — column math matches upstream's
s:text_begin, auto-indent handling sound, no parity gaps or regressions.

It flagged one low-severity item shared with the pre-existing smart_return:
neither guarded the completion popup. Upstream maps <CR>/<S-CR> as
`pumvisible() ? '<CR>' : …`. Added that guard to both smart_return and
smart_shift_return (both clients) so a <CR>/<S-CR> accepts the completion
instead of continuing the list when the popup is open.

Left as a documented minor divergence: <S-CR> on an *empty* bullet emits the
aligned no-marker continuation rather than upstream's "blank line above, keep
marker" (niche). Neovim 305, Vim 299/18/21 pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lua/nuwiki/commands.lua

@@ -963,6 +963,9 @@ function M.smart_return()
   local cr = tc('<CR>')
   local esc = tc('<Esc>')
 
+  -- Accept the completion (plain <CR>) when the popup menu is open, like upstream.
+  if vim.fn.pumvisible() == 1 then return cr end
+
   if is_table_row(line) then
     local row = vim.api.nvim_win_get_cursor(0)[1]
     return string.format(
@@ -1006,6 +1009,7 @@ end
 function M.smart_shift_return()
   local line = vim.api.nvim_get_current_line()
   local cr = vim.api.nvim_replace_termcodes('<CR>', true, false, true)
+  if vim.fn.pumvisible() == 1 then return cr end
   local indent, marker, after = line:match('^(%s*)([%-%*%#])%s(.*)$')
   if not marker then
     local n