feat(parity): close the entire P3 Mappings section

Both remaining mapping gaps (client-side, both clients):

- Visual <CR>: upstream binds it to NormalizeLinkVisualCR (== visual `+`).
  Added `x <CR>` -> normalize_link(true) (wrap the selection as a wikilink)
  in both clients.

- Insert <S-CR>: upstream's multiline-list-item continuation (VimwikiReturn
  2 2 -> kbd_cr with no new marker). New smart_shift_return (both clients,
  <expr> insert mapping): on a list item returns <CR> + spaces aligning under
  the item text (marker width + 1, +4 for a `[ ] ` checkbox), no marker; off a
  list item, a plain <CR>. Mirrors smart_return's auto-indent handling.

Tests: map.visual_cr_wraps_selection + map.insert_shift_cr_multiline
(test-keymaps.lua, the latter exercising the handler directly since <S-CR>
doesn't round-trip headless feedkeys), and map.visual_cr_wraps_selection +
cr.shift_cr_multiline{,_checkbox} (test-keymaps-vim.vim). Docs (README +
doc/nuwiki.txt) and the gap doc updated. Neovim 305, Vim 299/18/21 pass.

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

lua/nuwiki/commands.lua

@@ -1000,6 +1000,31 @@ function M.smart_return()
   return cr
 end
 
+-- Insert-mode `<S-CR>`: continue the current list item on a new line WITHOUT a
+-- new marker, indented to align under the item's text — vimwiki's "multiline
+-- list item" (`VimwikiReturn 2 2`). Off a list item it's a plain `<CR>`.
+function M.smart_shift_return()
+  local line = vim.api.nvim_get_current_line()
+  local cr = vim.api.nvim_replace_termcodes('<CR>', true, false, true)
+  local indent, marker, after = line:match('^(%s*)([%-%*%#])%s(.*)$')
+  if not marker then
+    local n
+    indent, n, marker, after = line:match('^(%s*)(%d+)([%.%)])%s(.*)$')
+    if marker then marker = n .. marker end
+  end
+  if not marker then
+    return cr
+  end
+  -- Align under the text: marker width + 1 space (+ 4 for a `[ ] ` checkbox).
+  local has_cb = after:match('^%[[%sxXoO%.%-]%]') ~= nil
+  local pad = #marker + 1 + (has_cb and 4 or 0)
+  -- Mirror smart_return's auto-indent handling: when an auto-indent mechanism
+  -- is active, Vim already re-inserts the line's indent after `<CR>`.
+  local auto = vim.bo.autoindent or vim.bo.smartindent or vim.bo.cindent
+      or (vim.bo.indentexpr ~= nil and vim.bo.indentexpr ~= '')
+  return cr .. (auto and '' or indent) .. string.rep(' ', pad)
+end
+
 -- :VimwikiReturn / :NuwikiReturn — the command form of the smart `<CR>`
 -- continuation (upstream's mapping-driven VimwikiReturn). Enters insert at end
 -- of line and triggers the buffer-local `<CR>` mapping (smart_return). Args