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>

scripts/test-keymaps.lua

@@ -401,6 +401,38 @@ vim.defer_fn(function()
     end
   end)
 
+  -- ===== Smart <CR> (Cluster 5) =====
+
+  run('cr.continues_list_marker', {
+    lines = { '- first' },
+    cursor = { 1, 7 }, -- end of line
+    keys = 'A<CR>second<Esc>',
+    expect_lines = { '- first', '- second' },
+    wait_ms = 100,
+  })
+  run('cr.continues_checkbox', {
+    lines = { '- [ ] first' },
+    cursor = { 1, 11 },
+    keys = 'A<CR>second<Esc>',
+    expect_lines = { '- [ ] first', '- [ ] second' },
+    wait_ms = 100,
+  })
+  run('cr.breaks_on_empty_list_line', {
+    lines = { '- ' },
+    cursor = { 1, 2 },
+    keys = 'A<CR>plain<Esc>',
+    expect_lines = { '', 'plain' },
+    wait_ms = 100,
+  })
+  run('cr.adds_new_table_row', {
+    lines = { '| a | b |' },
+    cursor = { 1, 4 }, -- inside `a`
+    keys = 'A<CR>x<Esc>',
+    -- New row inserted below with empty cells; cursor jumps to first.
+    expect_lines = { '| a | b |', '|x   |   |' },
+    wait_ms = 100,
+  })
+
   -- ===== Diary nav =====
   -- Tested via the LSP open-* responses; can't fully validate without
   -- a populated diary, but at least confirm the maps fire without