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>

lua/nuwiki/ftplugin.lua

@@ -9,34 +9,33 @@ local function setup_folding(bufnr, folding_mode)
   if folding_mode == 'off' then
     return
   end
-  if folding_mode == 'expr' then
-    vim.api.nvim_set_option_value('foldmethod', 'expr', { buf = bufnr })
-    vim.api.nvim_set_option_value(
-      'foldexpr',
-      'v:lua.require("nuwiki.folding").expr()',
-      { buf = bufnr }
-    )
-    vim.api.nvim_set_option_value(
-      'foldtext',
-      'v:lua.require("nuwiki.folding").foldtext()',
-      { buf = bufnr }
-    )
-    return
-  end
-  -- folding_mode == 'lsp' (default): rely on the server's
-  -- `foldingRange` capability. Neovim 0.11+ enables it automatically
-  -- via `vim.lsp.foldexpr`. Older versions get the regex fallback.
-  if vim.fn.has('nvim-0.11') == 1 and vim.lsp.foldexpr then
-    vim.api.nvim_set_option_value('foldmethod', 'expr', { buf = bufnr })
-    vim.api.nvim_set_option_value(
-      'foldexpr',
-      'v:lua.vim.lsp.foldexpr()',
-      { buf = bufnr }
-    )
-  else
-    -- Pre-0.11 — degrade to the regex fallback so users still get folds.
-    setup_folding(bufnr, 'expr')
+  -- `foldmethod` / `foldexpr` / `foldtext` are window-local options.
+  -- The ftplugin runs during BufRead → FileType, by which time the
+  -- buffer is displayed in the current window. `vim.opt_local` writes
+  -- to the current window's view of the option.
+  local apply = function()
+    if folding_mode == 'expr'
+       or not (vim.fn.has('nvim-0.11') == 1 and vim.lsp.foldexpr)
+    then
+      vim.opt_local.foldmethod = 'expr'
+      vim.opt_local.foldexpr = 'v:lua.require("nuwiki.folding").expr()'
+      vim.opt_local.foldtext = 'v:lua.require("nuwiki.folding").foldtext()'
+    else
+      -- LSP-backed folding via Neovim 0.11+'s built-in foldexpr.
+      vim.opt_local.foldmethod = 'expr'
+      vim.opt_local.foldexpr = 'v:lua.vim.lsp.foldexpr()'
+    end
   end
+
+  -- Set on the current window first (the one that triggered the
+  -- ftplugin). Also re-apply on any future split that views this
+  -- buffer, so `<C-w>v` / `:split` preserves the fold method.
+  apply()
+  vim.api.nvim_create_autocmd('BufWinEnter', {
+    buffer = bufnr,
+    group = vim.api.nvim_create_augroup('nuwiki_folding_' .. bufnr, { clear = true }),
+    callback = apply,
+  })
 end
 
 function M.attach(bufnr)

lua/nuwiki/init.lua

@@ -15,6 +15,20 @@ local lsp = require('nuwiki.lsp')
 
 function M.setup(opts)
   config.apply(opts or {})
+
+  -- Neovim 0.7+ resolves filetypes via `vim.filetype.match` before any
+  -- `ftdetect/*.vim` autocmd fires, and its bundled rule for `*.wiki`
+  -- returns `mediawiki`. Register an authoritative rule here so this
+  -- plugin's filetype (vimwiki) wins.
+  if vim.filetype and vim.filetype.add then
+    local extensions = { wiki = 'vimwiki' }
+    local ext = (config.options.file_extension or '.wiki'):gsub('^%.', '')
+    if ext ~= 'wiki' and ext ~= '' then
+      extensions[ext] = 'vimwiki'
+    end
+    vim.filetype.add({ extension = extensions })
+  end
+
   lsp.register()
 end