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>

doc/nuwiki.txt

@@ -207,6 +207,28 @@ Vim-specific globals ~
     When 1, the Vim path skips every buffer-local keymap. Equivalent to
     `mappings.enabled = false` in Neovim.
 
+                                              *g:nuwiki_no_wiki_prefix_mappings*
+                                                    *g:nuwiki_no_links_mappings*
+                                                    *g:nuwiki_no_lists_mappings*
+                                                  *g:nuwiki_no_headers_mappings*
+                                            *g:nuwiki_no_table_editing_mappings*
+                                                    *g:nuwiki_no_diary_mappings*
+                                              *g:nuwiki_no_html_export_mappings*
+                                             *g:nuwiki_no_text_objects_mappings*
+`g:nuwiki_no_<group>_mappings` 0
+    Per-subgroup opt-outs, each defaulting to 0. Setting one to 1 drops just
+    that group's buffer-local keymaps while leaving the rest in place; the
+    whole-layer |g:nuwiki_no_default_mappings| still wins over all of them.
+    These mirror the Lua-side `mappings.<group>` booleans. Valid groups:
+        `wiki_prefix`     `<Leader>w*`
+        `links`           `<CR>`, `<S-CR>`, `<Tab>`, `<BS>`, `+`, …
+        `lists`           `<C-Space>`, `gl*`, `o`/`O`, insert-mode list edits
+        `headers`         `=`, `-`, `]]`, `[[`, `]=`, `[=`, `]u`, `[u`
+        `table_editing`   `gqq`, `<A-Left>`, `<A-Right>`, insert `<Tab>`
+        `diary`           `<C-Down>`, `<C-Up>`
+        `html_export`     `<Leader>wh`, `<Leader>whh`, `<Leader>wha`
+        `text_objects`    `ah`, `ih`, `al`, `il`, `a\`, `i\`, `ac`, `ic`
+
                                                           *g:nuwiki_no_folding*
 `g:nuwiki_no_folding`          0
     When 1, the Vim path skips `foldexpr` / `foldmethod` setup.
@@ -378,8 +400,11 @@ Other ~
 ==============================================================================
 6. KEYMAPS                                                  *nuwiki-keymaps*
 
-Every keymap is buffer-local. Disable a group via `mappings.<group> =
-false` in Neovim, or `let g:nuwiki_no_default_mappings = 1` in Vim.
+Every keymap is buffer-local. Disable the whole layer via
+`mappings.enabled = false` in Neovim, or `let g:nuwiki_no_default_mappings
+= 1` in Vim. Drop a single group via `mappings.<group> = false` in Neovim,
+or `let g:nuwiki_no_<group>_mappings = 1` in Vim (see
+|g:nuwiki_no_links_mappings| for the group list).
 
 Links (normal mode) ~