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',