chore(scripts): add syntax-diag helper for debugging highlight issues

Drop-in script that dumps the syntax/highlight state of the current
buffer to /tmp/nuwiki-syndiag.log: filetype, b:current_syntax, every
syntax/vimwiki.vim found in &runtimepath (to spot duplicates from
other plugins), the resolved attributes of our highlight groups +
the colorscheme groups they link to, a per-segment synID walk over
the buffer, and the filtered scriptnames list. Useful when a user
reports broken highlighting and we need to know whether the patterns
aren't matching, the colorscheme is the culprit, or another plugin's
syntax/vimwiki.vim is winning the runtimepath race.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/syntax-diag.vim

@@ -0,0 +1,75 @@
+" scripts/syntax-diag.vim — dump highlighting state to /tmp/nuwiki-syndiag.log
+"
+" Usage: open a .wiki file with VARIED markup (heading, *bold*, _italic_,
+" `code`), then:
+"   :source scripts/syntax-diag.vim
+" Then paste /tmp/nuwiki-syndiag.log back.
+
+redir! > /tmp/nuwiki-syndiag.log
+
+echo '=== state ==='
+echo 'filetype=' . &filetype
+echo 'syntax=' . &syntax
+echo 'b:current_syntax=' . get(b:, 'current_syntax', '<unset>')
+echo 'syntax_on=' . exists('g:syntax_on')
+echo 'colors_name=' . get(g:, 'colors_name', '<unset>')
+echo 'background=' . &background
+echo 'termguicolors=' . &termguicolors
+echo 't_Co=' . &t_Co
+echo 'has(nvim)=' . has('nvim')
+echo 'g:loaded_nuwiki=' . get(g:, 'loaded_nuwiki', '<unset>')
+echo 'g:loaded_vimwiki=' . get(g:, 'loaded_vimwiki', '<unset>')
+
+echo ''
+echo '=== every syntax/vimwiki.vim in runtimepath (first wins) ==='
+for s:p in split(&runtimepath, ',')
+  let s:f = s:p . '/syntax/vimwiki.vim'
+  if filereadable(s:f)
+    echo s:f
+  endif
+endfor
+
+echo ''
+echo '=== :hi for our groups (NO silent — capture into redir) ==='
+for s:g in ['nuwikiHeading', 'nuwikiBold', 'nuwikiItalic', 'nuwikiCode',
+      \ 'nuwikiPlaceholder', 'nuwikiCommentLine', 'nuwikiWikilink',
+      \ 'Title', 'PreProc', 'String', 'Comment', 'Underlined', 'Normal']
+  execute 'highlight ' . s:g
+endfor
+
+echo ''
+echo '=== synID walk over current buffer (first 30 lines) ==='
+for s:ln in range(1, min([line('$'), 30]))
+  let s:l = getline(s:ln)
+  let s:segs = []
+  let s:last = ''
+  let s:start = 1
+  for s:c in range(1, len(s:l) + 1)
+    let s:g = s:c <= len(s:l) ? synIDattr(synIDtrans(synID(s:ln, s:c, 1)), 'name') : ''
+    if s:g != s:last
+      if s:start <= s:c - 1
+        let s:txt = strpart(s:l, s:start - 1, s:c - s:start)
+        call add(s:segs, s:last . ':' . s:txt)
+      endif
+      let s:last = s:g
+      let s:start = s:c
+    endif
+  endfor
+  echo printf('L%d %s', s:ln, join(s:segs, ' | '))
+endfor
+
+echo ''
+echo '=== :syntax list (full) ==='
+syntax list
+
+echo ''
+echo '=== :scriptnames (filtered) ==='
+let s:sn = execute('scriptnames')
+for s:line in split(s:sn, '\n')
+  if s:line =~? 'vimwiki\|nuwiki\|syntax\|colors'
+    echo s:line
+  endif
+endfor
+
+redir END
+echo 'Wrote /tmp/nuwiki-syndiag.log'