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>

doc/nuwiki.txt

@@ -352,11 +352,12 @@ Wiki / navigation ~
 
                                                               *:NuwikiIndex*
 :NuwikiIndex [{count}]
-    Open wiki N's index page (default: 1).
+    Open wiki N's index page (default: 1). Defined globally, so it works
+    from any buffer (not just inside a wiki).
 
                                                            *:NuwikiTabIndex*
 :NuwikiTabIndex [{count}]
-    Same, in a new tab.
+    Same, in a new tab. Also global.
 
                                                            *:NuwikiUISelect*
 :NuwikiUISelect
@@ -368,6 +369,19 @@ Wiki / navigation ~
     Echo the nuwiki version (`g:nuwiki_version`) and the host editor.
     Also available as `:VimwikiShowVersion`.
 
+                                                               *:NuwikiVar*
+:NuwikiVar [{key} [{value}]]
+    Get/set nuwiki client config. No argument prints the config (Vim:
+    every `g:nuwiki_*`; Neovim: the resolved |nuwiki-setup| options); one
+    argument prints that key; two or more sets it (server-backed settings
+    need a restart to take effect). Also `:VimwikiVar`.
+
+                                                            *:NuwikiReturn*
+:NuwikiReturn
+    Command form of the smart `<CR>` continuation: continue the current
+    list item / table row. Mostly used internally; the insert-mode `<CR>`
+    mapping does the same. Also `:VimwikiReturn`.
+
                                                                *:NuwikiGoto*
 :NuwikiGoto {page}
     Open `{page}.wiki` by name.