feat(parity): close the entire P3 Commands section

All six remaining command gaps (client-side; no server change):

- VimwikiRenameFile: bare -> -nargs=? (arg accepted-ignored; rename still
  delegates to the LSP rename prompt). No more E488.

- VimwikiVar / NuwikiVar: get/set the client config (upstream vimwiki#vars#cmd).
  No arg prints config (Vim: g:nuwiki_*; Neovim: resolved setup() opts), one arg
  gets a key, two+ sets it. Global command, both clients.

- VimwikiReturn / NuwikiReturn: command form of the smart <CR> continuation —
  enters insert at EOL and fires the <CR> mapping (return_cmd / vimwiki_return).
  Upstream's mode-flag args accepted but unused.

- VimwikiTableAlignQ vs AlignW: corrected — upstream's gqq/gww are identical on
  a table; the only difference is the non-table fallback (native `normal! gqq`
  vs `gww`). New table_align_or_cmd(cmd) aligns on a table row, else runs
  `normal! <cmd>`. Q/gqq/gq1 -> gqq; W/gww/gw1 -> gww; NuwikiTableAlign -> gqq.

- VimwikiSearch / VWS: scope the lvimgrep to the current wiki's root (resolved
  client-side) over <root>/**/*<ext>, not CWD `**`; empty pattern reuses the
  last search. Both clients.

- VimwikiIndex family: add global Vimwiki/NuwikiIndex + TabIndex entry points
  (plugin/nuwiki.vim + init.lua _setup_global_commands, with -count), so a wiki
  opens from any buffer. Buffer-local copies still shadow inside wiki buffers.

Tests: surface.{Vimwiki,Nuwiki}{Return,Var}, cmd.VimwikiVar_sets_*,
cmd.global_entry_points (both harnesses). Docs (README + doc/nuwiki.txt) and the
gap doc updated. Neovim 299, Vim 291/18/21 pass.

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

development/tests/test-keymaps.lua

@@ -139,7 +139,7 @@ vim.defer_fn(function()
     'RebuildTags', '2HTML', '2HTMLBrowse', 'All2HTML', 'Rss',
     'NormalizeLink', 'RenumberList', 'RenumberAllLists',
     'ListToggle', 'IncrementListItem', 'DecrementListItem',
-    'CatUrl', 'ShowVersion',
+    'CatUrl', 'ShowVersion', 'Return', 'Var',
   }
   for _, suffix in ipairs(both_forms) do
     for _, prefix in ipairs({ 'Nuwiki', 'Vimwiki' }) do
@@ -283,6 +283,27 @@ vim.defer_fn(function()
     vim.cmd('bwipeout!')
   end
 
+  -- :VimwikiVar key val sets the in-memory option.
+  do
+    vim.cmd('VimwikiVar testkey hello')
+    local v = require('nuwiki.config').options.testkey
+    record(v == 'hello', 'cmd.VimwikiVar_sets_option', 'options.testkey=' .. tostring(v))
+  end
+
+  -- Index / TabIndex / Var / ShowVersion are global (work outside a wiki
+  -- buffer). Open a throwaway scratch buffer and confirm they're visible.
+  do
+    vim.cmd('enew')
+    vim.bo.buftype = 'nofile'
+    local ok = vim.fn.exists(':VimwikiIndex') == 2
+      and vim.fn.exists(':VimwikiTabIndex') == 2
+      and vim.fn.exists(':VimwikiVar') == 2
+      and vim.fn.exists(':VimwikiShowVersion') == 2
+    record(ok, 'cmd.global_entry_points',
+      ok and '' or 'a global command is not visible outside a wiki buffer')
+    vim.cmd('bwipeout!')
+  end
+
   -- ===== Lists =====
 
   run('list.toggle_via_C-Space', {