feat(commands): VimwikiListChangeLvl -range -nargs=+ (P3 parity)

Upstream is `-range -nargs=+` (change_level(line1, line2, direction,
plus_children)); nuwiki was -nargs=? and range-less, so `:1,3VimwikiListChangeLvl
increase 0` raised E481/E488 and the level change never spanned a selection.

All four defs (Vim+Neovim × Vimwiki+Nuwiki) are now -range -nargs=+ and forward
<line1>, <line2>, <f-args>. list_change_lvl(line1, line2, direction,
[plus_children]) (both clients) parses the required direction + optional subtree
flag and applies the change to every list item in the range via the existing
over_range helper.

Tests: cmd.ListChangeLvl_range_indents (test-keymaps.lua) +
cmd.VimwikiListChangeLvl_accepts_range_and_args (test-keymaps-vim.vim).

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

lua/nuwiki/commands.lua

@@ -585,11 +585,14 @@ function M.list_change_level(delta, whole_subtree)
   exec('nuwiki.list.changeLevel', { args })
 end
 
--- Vimwiki's `:VimwikiListChangeLvl decrease|increase 0` keeps a single
--- entry point. Forward to changeLevel.
-function M.list_change_lvl(direction)
+-- `:[range]VimwikiListChangeLvl {increase|decrease} [plus_children]` — upstream
+-- is `-range -nargs=+`. Direction is required; the optional second arg cascades
+-- to each item's subtree; a range applies it to every list item in the range.
+function M.list_change_lvl(line1, line2, direction, plus_children)
   local delta = (direction == 'increase' or direction == 'indent') and 1 or -1
-  M.list_change_level(delta, false)
+  local n = tonumber(plus_children)
+  local children = n ~= nil and n ~= 0
+  over_range(line1, line2, function() M.list_change_level(delta, children) end)
 end
 
 --- Cycle the list marker forward (`direction = 1`) or backward