feat(client): add per-subgroup Vim mapping opt-out globals

The plain-Vim ftplugin only exposed a whole-layer
g:nuwiki_no_default_mappings gate, so Vim users could not drop a single
keymap group the way Neovim users can via mappings.<group> = false. Add
g:nuwiki_no_{wiki_prefix,links,lists,headers,table_editing,diary,
html_export,text_objects}_mappings, each wrapping its group block, to
reach parity with lua/nuwiki/keymaps.lua. Cover them with a dedicated
opt-out harness and document them in the README and help file.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

development/tests/test-keymaps-vim-optout.vim

@@ -0,0 +1,73 @@
+" development/tests/test-keymaps-vim-optout.vim — driven by
+" development/tests/test-keymaps-vim.sh.
+"
+" Per-subgroup mapping opt-out regression. The shell driver sets a subset
+" of `g:nuwiki_no_<group>_mappings = 1` globals *before* the ftplugin loads,
+" so this harness asserts that the disabled groups leave no buffer-local
+" mappings while the still-enabled groups keep theirs. Mirrors the Lua-side
+" subgroup toggles in lua/nuwiki/keymaps.lua.
+
+let s:results = []
+let s:pass = 0
+let s:fail = 0
+let s:out = $NUWIKI_KEYMAP_RESULTS
+
+function! s:record(ok, name, detail) abort
+  let l:line = (a:ok ? 'PASS' : 'FAIL') . ' ' . a:name
+  if a:detail !=# ''
+    let l:line .= ' — ' . a:detail
+  endif
+  call add(s:results, l:line)
+  if a:ok
+    let s:pass += 1
+  else
+    let s:fail += 1
+  endif
+endfunction
+
+function! s:has_map(lhs, mode) abort
+  let l:d = maparg(a:lhs, a:mode, 0, 1)
+  return !empty(l:d) && get(l:d, 'buffer', 0)
+endfunction
+
+" Groups the driver disabled (g:nuwiki_no_<group>_mappings = 1): every
+" representative LHS below must be ABSENT.
+let s:disabled = [
+      \ ['wiki_prefix', '<Leader>ww', 'n'],
+      \ ['wiki_prefix', '<Leader>w<Leader>w', 'n'],
+      \ ['links', '<CR>', 'n'],
+      \ ['links', '+', 'n'],
+      \ ['links', '<Leader>wr', 'n'],
+      \ ['headers', '=', 'n'],
+      \ ['headers', ']]', 'n'],
+      \ ['headers', ']u', 'n'],
+      \ ['text_objects', 'ah', 'x'],
+      \ ['text_objects', 'il', 'o'],
+      \ ]
+for s:e in s:disabled
+  let s:ok = !s:has_map(s:e[1], s:e[2])
+  call s:record(s:ok ? 1 : 0, 'optout.' . s:e[0] . '.' . s:e[2] . '.' . s:e[1],
+        \ s:ok ? '' : s:e[1] . ' (' . s:e[2] . ') still mapped despite opt-out')
+endfor
+
+" Groups left enabled: every representative LHS must still be PRESENT.
+let s:enabled = [
+      \ ['lists', 'gln', 'n'],
+      \ ['lists', 'gnt', 'n'],
+      \ ['lists', '<C-Space>', 'n'],
+      \ ['diary', '<C-Down>', 'n'],
+      \ ['table_editing', 'gqq', 'n'],
+      \ ['table_editing', '<A-Left>', 'n'],
+      \ ['html_export', '<Leader>wh', 'n'],
+      \ ['html_export', '<Leader>wha', 'n'],
+      \ ]
+for s:e in s:enabled
+  let s:ok = s:has_map(s:e[1], s:e[2])
+  call s:record(s:ok ? 1 : 0, 'kept.' . s:e[0] . '.' . s:e[2] . '.' . s:e[1],
+        \ s:ok ? '' : s:e[1] . ' (' . s:e[2] . ') unexpectedly absent')
+endfor
+
+call add(s:results, '')
+call add(s:results, printf('SUMMARY: %d passed, %d failed', s:pass, s:fail))
+call writefile(s:results, s:out)
+execute (s:fail == 0 ? 'qall!' : 'cquit!')