feat(config): vimwiki drop-in config + global per-wiki defaults

Two related config-ergonomics features for vimwiki migrants, sharing the
same scalar-global machinery.

1. Upstream g:vimwiki_* drop-in (Vim client). When nuwiki isn't configured
   natively, the Vim client reads g:vimwiki_list + g:vimwiki_* globals and
   translates them into nuwiki's schema: per-wiki path->root,
   path_html->html_path, template_*, css_name, auto_export/auto_toc,
   syntax, ext->file_extension, index, diary_*, name. g:nuwiki_* always
   wins. Lets a vimwiki user drop in nuwiki without rewriting config.

2. Global per-wiki defaults (both clients). A display/generation setting
   given once at the top level — g:nuwiki_<key> / setup({<key>=…}) /
   g:vimwiki_<key> — is folded into every wiki as a default; a per-wiki
   value overrides it (vimwiki's model). Covers toc_header(_level),
   toc_link_format, links_header(_level), tags_header(_level),
   html_header_numbering(_sym), links_space_char, list_margin, listsyms,
   listsym_rejected, and the auto_* toggles. Only values the user
   explicitly set fold — built-in defaults don't (added config.user
   tracking on the Lua side so a default toc_header_level=1 isn't pushed
   onto every wiki). User config tables are never mutated.

Both clients kept in lock-step (config-parity golden enforces identical
payloads). New harnesses test-vimwiki-compat-vim (18) and
test-global-shorthand (8), wired into CI. Server test
vimwiki_compat_payload_sets_toc_level_per_wiki. Docs in README +
known-issues.md (drop-in is currently Vim-only). Rust 573 passed, clippy
clean, all harnesses green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lua/nuwiki/config.lua

@@ -103,7 +103,14 @@ M.defaults = {
 
 M.options = vim.deepcopy(M.defaults)
 
+-- The raw table the user passed to setup(), kept separate from the
+-- defaults-merged `options`. Used to tell an explicitly-set value apart from
+-- a built-in default — e.g. so the LSP client only treats a per-wiki key as a
+-- global default when the user actually set it.
+M.user = {}
+
 function M.apply(user)
+  M.user = user or {}
   M.options = vim.tbl_deep_extend('force', M.defaults, user or {})
 end
 

lua/nuwiki/lsp.lua

@@ -14,6 +14,39 @@ local function command()
   return { install.expected_path() }
 end
 
+-- Per-wiki settings that double as wiki-wide defaults (vimwiki-style global
+-- shorthand): set once at the top level — via setup() or `g:nuwiki_<key>` —
+-- and every wiki inherits it; a per-wiki value overrides. Mirrors the Vim
+-- client's `s:scalar_global_map`.
+local SCALAR_GLOBALS = {
+  'toc_header', 'toc_header_level', 'toc_link_format',
+  'links_header', 'links_header_level',
+  'tags_header', 'tags_header_level',
+  'html_header_numbering', 'html_header_numbering_sym',
+  'links_space_char', 'list_margin',
+  'listsyms', 'listsym_rejected',
+  'auto_export', 'auto_toc',
+  'auto_generate_links', 'auto_generate_tags', 'auto_diary_index',
+}
+
+-- Resolve the global scalar defaults from config the user *explicitly* set:
+-- the raw setup() table first, then the `g:nuwiki_<key>` global. Built-in
+-- defaults are deliberately excluded so they don't get folded into every wiki
+-- (that would diverge from the Vim client and override per-wiki values with a
+-- default).
+local function scalar_globals()
+  local user = config.user or {}
+  local g = {}
+  for _, key in ipairs(SCALAR_GLOBALS) do
+    if user[key] ~= nil then
+      g[key] = user[key]
+    elseif vim.g['nuwiki_' .. key] ~= nil then
+      g[key] = vim.g['nuwiki_' .. key]
+    end
+  end
+  return g
+end
+
 -- Merge g:nuwiki_wikis (VimL config) into opts.wikis when the user has not
 -- passed wikis through setup(). This lets users who configure via a .vim
 -- file avoid duplicating the list in their Lua setup() call.
@@ -22,6 +55,23 @@ local function resolved_opts()
   if not opts.wikis and vim.g.nuwiki_wikis then
     opts = vim.tbl_extend('force', opts, { wikis = vim.g.nuwiki_wikis })
   end
+  local globals = scalar_globals()
+  if vim.tbl_isempty(globals) then
+    return opts
+  end
+  if opts.wikis then
+    -- Fold the globals into each wiki as defaults (per-wiki value wins),
+    -- without mutating the user's setup table.
+    local folded = {}
+    for i, w in ipairs(opts.wikis) do
+      folded[i] = vim.tbl_extend('force', globals, w)
+    end
+    opts = vim.tbl_extend('force', opts, { wikis = folded })
+  else
+    -- Single-wiki shorthand: ensure the globals (incl. the g:nuwiki_* form)
+    -- sit at the top level for the server's single-wiki desugaring.
+    opts = vim.tbl_extend('keep', opts, globals)
+  end
   return opts
 end