fix(syntax): highlight code blocks with embedded language syntax

`{{{lang … }}}` fences were painted as one String group. Add automatic
nested-syntax highlighting (like vimwiki): scan the buffer for the languages
used, `:syntax include` each under a cluster, and define a contained region
per language so e.g. `{{{sh` blocks get shell highlighting and `{{{python`
gets Python. Bare `{{{` / `{{{class="…"` fences fall back to nuwikiPreformatted.
A small alias map maps labels like bash/shell → sh.

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

syntax/vimwiki.vim

@@ -75,7 +75,7 @@ syntax match nuwikiHeading6      /^\s*======\s.\{-}\s======\s*$/
 syntax match nuwikiHorizontal   /^-\{4,}\s*$/
 syntax match nuwikiCommentLine  /^%%.*$/
 syntax region nuwikiCommentBlock start=/^%%+/ end=/+%%/ keepend
-syntax region nuwikiPreformatted start=/^{{{/ end=/^}}}/ keepend
+syntax region nuwikiPreformatted matchgroup=nuwikiPreDelim start=/^\s*{{{/ end=/^\s*}}}\s*$/ keepend
 syntax region nuwikiMathBlock    start=/^{{\$/ end=/^}}\$/ keepend
 syntax match nuwikiPlaceholder  /^%\(title\|nohtml\|template\|date\)\>.*$/
 
@@ -117,6 +117,7 @@ highlight default link nuwikiHorizontal     Comment
 highlight default link nuwikiCommentLine    Comment
 highlight default link nuwikiCommentBlock   Comment
 highlight default link nuwikiPreformatted   String
+highlight default link nuwikiPreDelim       Comment
 highlight default link nuwikiMathBlock      Number
 highlight default link nuwikiPlaceholder    PreProc
 " Bold / Italic use explicit attributes so they work in plain Vim too (no
@@ -134,4 +135,74 @@ highlight default link nuwikiWikilink       Underlined
 highlight default link nuwikiTransclusion   Identifier
 highlight default link nuwikiUrl            Underlined
 
+" ===== Nested syntax for code blocks =====
+"
+" Highlight `{{{lang … }}}` fences with the embedded language's syntax, like
+" vimwiki's automatic nested syntaxes. We scan the buffer for the languages
+" actually used, `:syntax include` each one under a cluster, then define a
+" contained region per language. Defined *after* nuwikiPreformatted so the
+" language regions win for tagged fences (Vim gives later items priority),
+" while bare `{{{` / `{{{class="…"` fences fall back to nuwikiPreformatted.
+
+" Labels that don't match a Vim filetype name 1:1.
+let s:nuwiki_nested_aliases = {
+      \ 'bash': 'sh', 'shell': 'sh', 'zsh': 'sh', 'console': 'sh',
+      \ 'js': 'javascript', 'ts': 'typescript', 'py': 'python',
+      \ 'rs': 'rust', 'yml': 'yaml', 'md': 'markdown', 'c++': 'cpp',
+      \ }
+
+" The language label of a fence line: the first whitespace-separated token
+" after `{{{` that isn't a `key=val` attribute (mirrors the Rust lexer).
+function! s:nuwiki_fence_label(line) abort
+  let l:after = matchstr(a:line, '^\s*{{{\zs.*$')
+  for l:tok in split(l:after, '\s\+')
+    if l:tok !~# '='
+      return l:tok
+    endif
+  endfor
+  return ''
+endfunction
+
+function! s:nuwiki_setup_nested() abort
+  let l:included = {}
+  let l:defined = {}
+  for l:lnum in range(1, line('$'))
+    let l:line = getline(l:lnum)
+    if l:line !~# '^\s*{{{'
+      continue
+    endif
+    let l:label = s:nuwiki_fence_label(l:line)
+    if l:label ==# '' || has_key(l:defined, l:label)
+      continue
+    endif
+    let l:defined[l:label] = 1
+    let l:ft = get(s:nuwiki_nested_aliases, tolower(l:label), tolower(l:label))
+    let l:cluster = 'nuwikiNested_' . substitute(l:ft, '[^a-zA-Z0-9]', '_', 'g')
+    if !has_key(l:included, l:cluster)
+      if exists('b:current_syntax')
+        unlet b:current_syntax
+      endif
+      try
+        execute 'syntax include @' . l:cluster . ' syntax/' . l:ft . '.vim'
+      catch
+        " No syntax file for this language — leave it to nuwikiPreformatted.
+        continue
+      endtry
+      if exists('b:current_syntax')
+        unlet b:current_syntax
+      endif
+      let l:included[l:cluster] = 1
+    endif
+    let l:region = 'nuwikiNestedRegion_' . substitute(l:label, '[^a-zA-Z0-9]', '_', 'g')
+    execute 'syntax region ' . l:region
+          \ . ' matchgroup=nuwikiPreDelim'
+          \ . ' start=/^\s*{{{\s*' . escape(l:label, '/\.*$^~[]') . '\>.*$/'
+          \ . ' end=/^\s*}}}\s*$/'
+          \ . ' contains=@' . l:cluster
+          \ . ' keepend'
+  endfor
+endfunction
+
+call s:nuwiki_setup_nested()
+
 let b:current_syntax = 'vimwiki'