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>

lua/nuwiki/keymaps.lua

@@ -254,6 +254,11 @@ function M.attach(bufnr, mappings)
         { desc = 'nuwiki: cycle list symbol backward (insert)' }, bufnr)
     map('i', '<C-L><C-M>', cmd.list_toggle_or_add_checkbox,
         { desc = 'nuwiki: toggle/add checkbox (insert)' }, bufnr)
+
+    -- Smart <CR>: continue list / table rows. `<expr>` so the function
+    -- result is the keystrokes fed — keeps undo coherent.
+    map('i', '<CR>', cmd.smart_return,
+        { desc = 'nuwiki: smart return (insert)', expr = true }, bufnr)
   end
 
   -- ===== Header levels + nav =====