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.sh

@@ -59,7 +59,44 @@ fi
 
 cat "$RESULTS"
 
-if grep -qE '^SUMMARY: [0-9]+ passed, 0 failed' "$RESULTS"; then
+if ! grep -qE '^SUMMARY: [0-9]+ passed, 0 failed' "$RESULTS"; then
+  exit 1
+fi
+
+# ===== Per-subgroup opt-out run =====
+# Fresh Vim instance with a subset of g:nuwiki_no_<group>_mappings set
+# *before* the ftplugin loads, so we can assert the disabled groups drop
+# their bindings while the rest survive.
+
+VIMRC_OPTOUT="$TMP/vimrc-optout"
+cat > "$VIMRC_OPTOUT" <<EOF
+set nocompatible
+let &runtimepath = '$REPO_ROOT' . ',' . &runtimepath
+let g:nuwiki_no_wiki_prefix_mappings = 1
+let g:nuwiki_no_links_mappings = 1
+let g:nuwiki_no_headers_mappings = 1
+let g:nuwiki_no_text_objects_mappings = 1
+filetype plugin indent on
+syntax enable
+EOF
+
+RESULTS_OPTOUT="$TMP/results-optout.txt"
+
+log "running opt-out harness…"
+NUWIKI_KEYMAP_RESULTS="$RESULTS_OPTOUT" \
+  timeout 30 vim -e -s -u "$VIMRC_OPTOUT" -c "edit $SEED" -c "source $REPO_ROOT/development/tests/test-keymaps-vim-optout.vim" \
+    </dev/null >"$TMP/vim-optout.log" 2>&1 || true
+
+if [[ ! -f "$RESULTS_OPTOUT" ]]; then
+  echo 'keymap opt-out harness produced no output' >&2
+  echo '--- vim log ---' >&2
+  cat "$TMP/vim-optout.log" >&2 || true
+  exit 1
+fi
+
+cat "$RESULTS_OPTOUT"
+
+if grep -qE '^SUMMARY: [0-9]+ passed, 0 failed' "$RESULTS_OPTOUT"; then
   exit 0
 fi
 exit 1