test(client): cover Vim/Neovim link-follow command surface + tab-drop

Add SplitLink/VSplitLink/TabnewLink/TabDropLink/GoBackLink to both
keymap harness command lists, plus a smoke check that
nuwiki.commands.follow_link_drop is callable.

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

development/tests/test-keymaps-vim.vim

@@ -103,6 +103,7 @@ endif
 
 let s:both_forms = [
       \ 'Index', 'TabIndex', 'UISelect', 'Goto', 'FollowLink', 'Backlinks',
+      \ 'SplitLink', 'VSplitLink', 'TabnewLink', 'TabDropLink', 'GoBackLink',
       \ 'NextLink', 'PrevLink', 'BaddLink', 'RenameFile', 'DeleteFile',
       \ 'MakeDiaryNote', 'TabMakeDiaryNote', 'MakeYesterdayDiaryNote',
       \ 'MakeTomorrowDiaryNote',

development/tests/test-keymaps.lua

@@ -130,6 +130,7 @@ vim.defer_fn(function()
   -- nuwiki-only maintenance command with no :Vimwiki* alias.
   local both_forms = {
     'Index', 'TabIndex', 'UISelect', 'Goto', 'FollowLink', 'Backlinks',
+    'SplitLink', 'VSplitLink', 'TabnewLink', 'TabDropLink', 'GoBackLink',
     'NextLink', 'PrevLink', 'BaddLink', 'RenameFile', 'DeleteFile',
     'MakeDiaryNote', 'TabMakeDiaryNote', 'MakeYesterdayDiaryNote',
     'MakeTomorrowDiaryNote',
@@ -167,6 +168,11 @@ vim.defer_fn(function()
     local exists = vim.fn.exists(':' .. cmd) == 2
     record(exists, 'surface.' .. cmd, exists and '' or (':' .. cmd .. ' not registered'))
   end
+  do
+    local callable = type(require('nuwiki.commands').follow_link_drop) == 'function'
+    record(callable, 'direct.follow_link_drop_callable',
+      callable and '' or 'nuwiki.commands.follow_link_drop is not a function')
+  end
 
   -- ===== Pure-Lua command invocation (cursor movement) =====
   -- :NuwikiNextLink / :NuwikiPrevLink wrap the link-search motion and run