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>

development/vimwiki-gap.md

@@ -183,7 +183,15 @@ fix site.
   cols/rows). Covered by `cmd.VimwikiTable_passes_cols_and_rows`
   (`test-keymaps.lua`, asserts a `:VimwikiTable 4 3` → 4-col, 3-row table) and
   `cmd.VimwikiTable_accepts_cols_rows` (`test-keymaps-vim.vim`, no E488).
-- [ ] `VimwikiListChangeLvl` is `-nargs=?` vs upstream `-range -nargs=+`.
+- [x] `VimwikiListChangeLvl` was `-nargs=?` (range-less) vs upstream
+  `-range -nargs=+`. _Fix:_ all four defs are now `-range -nargs=+` passing
+  `<line1>, <line2>, <f-args>`; `list_change_lvl(line1, line2, direction,
+  [plus_children])` (both clients) parses the required direction + optional
+  subtree flag and applies the level change to every list item in the range
+  via the existing `over_range` helper. Covered by
+  `cmd.ListChangeLvl_range_indents` (`test-keymaps.lua`) and
+  `cmd.VimwikiListChangeLvl_accepts_range_and_args` (`test-keymaps-vim.vim`,
+  no E481/E488).
 - [x] `VimwikiRebuildTags` `-bang` ("rebuild all") — `:…RebuildTags!` now passes
   `{ all: true }`; the server (`tags_rebuild`) re-indexes **every** configured
   wiki (via `wikis_snapshot()`) instead of just the current one. `-bang` added to