fix(ftplugin): load on .wiki buffers without manual filetype override

Three reasons the plugin wasn't activating on `nvim --clean -u init.lua
foo.wiki`:

1. **File-name mismatch.** Vim's filetype-plugin runtime looks for
   `ftplugin/<filetype>.vim` matching the *filetype name*. Our
   filetype is `vimwiki` (per `ftdetect`) but the file was named
   `ftplugin/nuwiki.vim` — so `:runtime! ftplugin/vimwiki.vim` from
   the built-in FileType autocmd never found it, and none of the
   `:Vimwiki*` / `:Nuwiki*` commands got defined. Same wart on the
   syntax side. Renamed:
   - `ftplugin/nuwiki.vim`  → `ftplugin/vimwiki.vim`
   - `syntax/nuwiki.vim`    → `syntax/vimwiki.vim`

2. **`setfiletype` deferred to Neovim's bundled rule.** Neovim ships
   a default `*.wiki → mediawiki` rule via `vim.filetype.add`, and
   `:setfiletype vimwiki` is a no-op once a filetype has been set.
   - `ftdetect/nuwiki.vim` now uses `set filetype=vimwiki` (force).
   - `lua/nuwiki/init.lua`'s `setup()` also calls
     `vim.filetype.add({ extension = { wiki = 'vimwiki', ... } })`
     so the modern table-based detection picks us before the
     bundled rule fires. Honours `config.options.file_extension`
     so a user with a custom extension gets covered automatically.

3. **`foldmethod`/`foldexpr` are window-local options.**
   `ftplugin.lua` was setting them with `nvim_set_option_value({ buf
   = bufnr })`, which threw `'buf' cannot be passed for window-local
   option 'foldmethod'` and aborted the rest of the per-buffer
   attach. Switched to `vim.opt_local.*` (window-aware), applied on
   the current window when the ftplugin fires, and re-applied via a
   `BufWinEnter` autocmd so `:split` / `:vsplit` keep the fold mode.

Verified end-to-end with:

  nvim --clean -u init.lua sample.wiki
  → filetype = vimwiki
  → :VimwikiTOC, :NuwikiIndex, … all defined
  → foldmethod = expr, foldexpr = v:lua.vim.lsp.foldexpr()
  → ftdetect, plugin, ftplugin, syntax all in :scriptnames

Total 377 tests still pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

syntax/vimwiki.vim

@@ -0,0 +1,89 @@
+" syntax/nuwiki.vim — default highlight groups for nuwiki.
+"
+" Two responsibilities:
+"   1. Default `@vimwiki*` highlight groups for the LSP semantic tokens
+"      (SPEC §11 P7). Loaded eagerly so the very first hover/highlight
+"      cycle after server startup already has colours.
+"   2. A small regex-based fallback so static highlighting works on
+"      buffers where the LSP isn't running yet.
+
+if exists('b:current_syntax')
+  finish
+endif
+
+" ===== Semantic-token defaults (Neovim's TS-style @group names) =====
+
+" Headings — level modifiers map to progressively softer accents.
+highlight default link @vimwikiHeading              Title
+highlight default link @vimwikiHeading.level1       Title
+highlight default link @vimwikiHeading.level2       Function
+highlight default link @vimwikiHeading.level3       Identifier
+highlight default link @vimwikiHeading.level4       Type
+highlight default link @vimwikiHeading.level5       Constant
+highlight default link @vimwikiHeading.level6       PreProc
+highlight default link @vimwikiHeading.centered     Title
+
+" Inline text styling.
+highlight default link @vimwikiBold                 markdownBold
+highlight default link @vimwikiItalic               markdownItalic
+highlight default link @vimwikiBoldItalic           markdownBoldItalic
+highlight default link @vimwikiStrikethrough        Comment
+highlight default link @vimwikiSuperscript          Number
+highlight default link @vimwikiSubscript            Number
+
+" Code + math.
+highlight default link @vimwikiCode                 String
+highlight default link @vimwikiCodeBlock            String
+highlight default link @vimwikiMath                 Number
+highlight default link @vimwikiMathBlock            Number
+
+" Links + transclusions.
+highlight default link @vimwikiWikilink             Underlined
+highlight default link @vimwikiExternalLink         Underlined
+highlight default link @vimwikiUrl                  Underlined
+highlight default link @vimwikiTransclusion         Identifier
+
+" Misc.
+highlight default link @vimwikiKeyword              Todo
+highlight default link @vimwikiColor                Constant
+highlight default link @vimwikiComment              Comment
+highlight default link @vimwikiDefinitionTerm       Statement
+
+" ===== Static fallback (no LSP required) =====
+
+syntax case match
+
+syntax match nuwikiHeading      /^\s*=\{1,6}\s.\{-}\s=\{1,6}\s*$/
+syntax match nuwikiHorizontal   /^-\{4,}\s*$/
+syntax match nuwikiCommentLine  /^%%.*$/
+syntax region nuwikiCommentBlock start=/^%%+/ end=/+%%/ keepend
+syntax region nuwikiPreformatted start=/^{{{/ end=/^}}}/ keepend
+syntax region nuwikiMathBlock    start=/^{{\$/ end=/^}}\$/ keepend
+syntax match nuwikiPlaceholder  /^%\(title\|nohtml\|template\|date\)\>.*$/
+
+syntax match nuwikiBold         /\*[^*]\+\*/
+syntax match nuwikiItalic       /\<_[^_]\+_\>/
+syntax match nuwikiCode         /`[^`]\+`/
+syntax match nuwikiMathInline   /\$[^$]\+\$/
+syntax match nuwikiKeyword      /\<\(TODO\|DONE\|STARTED\|FIXME\|FIXED\|XXX\)\>/
+syntax region nuwikiWikilink    start=/\[\[/ end=/\]\]/ keepend
+syntax region nuwikiTransclusion start=/{{[^{]/ end=/}}/ keepend
+syntax match nuwikiUrl          /\<\(https\?\|ftp\|mailto\|file\):\/\?\/\?\S\+/
+
+highlight default link nuwikiHeading        Title
+highlight default link nuwikiHorizontal     Comment
+highlight default link nuwikiCommentLine    Comment
+highlight default link nuwikiCommentBlock   Comment
+highlight default link nuwikiPreformatted   String
+highlight default link nuwikiMathBlock      Number
+highlight default link nuwikiPlaceholder    PreProc
+highlight default link nuwikiBold           Bold
+highlight default link nuwikiItalic         Italic
+highlight default link nuwikiCode           String
+highlight default link nuwikiMathInline     Number
+highlight default link nuwikiKeyword        Todo
+highlight default link nuwikiWikilink       Underlined
+highlight default link nuwikiTransclusion   Identifier
+highlight default link nuwikiUrl            Underlined
+
+let b:current_syntax = 'vimwiki'