parity(5): smart <CR> in insert mode (VimwikiReturn)

Bind `<CR>` in insert mode as an `<expr>` map to a `smart_return`
helper. Behaviour matches upstream vimwiki's `:VimwikiReturn`:

  - On a list line with content → continue the list with the same
    marker on a new line (preserving leading checkbox `[ ]` if any).
  - On an empty list line (marker only) → clear the marker and break
    out of the list with a plain newline.
  - Inside a `|…|` table row → insert a fresh empty row below with
    the same column count, cursor lands inside the first cell.
  - Otherwise → plain `<CR>`.

Implementation note: the helper is `<expr>`-bound, which means
textlock is active and the buffer can't be mutated from inside the
callback. The function returns key sequences only — including
`<Esc>0DA<CR>` for the empty-list break and `<Esc>0li` for the
table-row cursor jump — so every effect flows through Vim's normal
keystroke pipeline and stays undo-coherent.

Both Lua and VimL counterparts shipped.

Tests: 4 new in scripts/test-keymaps.lua covering list continuation,
checkbox preservation, empty-marker break-out, and new table row.

Gates: 421 Rust / 35 Neovim / 12 Vim.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ftplugin/vimwiki.vim

@@ -175,6 +175,7 @@ if !has('nvim')
     inoremap <silent><buffer> <C-L><C-J> <C-o>:call nuwiki#commands#list_cycle_symbol(1)<CR>
     inoremap <silent><buffer> <C-L><C-K> <C-o>:call nuwiki#commands#list_cycle_symbol(-1)<CR>
     inoremap <silent><buffer> <C-L><C-M> <C-o>:call nuwiki#commands#list_toggle_or_add_checkbox()<CR>
+    inoremap <silent><buffer><expr> <CR> nuwiki#commands#smart_return()
 
     " Headers
     nnoremap <silent><buffer> =          :call nuwiki#commands#heading_add()<CR>