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>

development/tests/test-global-shorthand.lua

@@ -0,0 +1,54 @@
+-- development/tests/test-global-shorthand.lua — Neovim client global
+-- shorthand: a per-wiki setting given once at the top level (setup() or
+-- g:nuwiki_<key>) is folded into every wiki as a default, and a per-wiki
+-- value overrides it. Mirror of the Vim test-vimwiki-compat checks.
+--
+-- Writes PASS/FAIL lines to $NUWIKI_GS_OUT; the wrapper fails on any FAIL.
+
+local config = require('nuwiki.config')
+local lsp = require('nuwiki.lsp')
+
+local out = {}
+local function check(desc, cond)
+  table.insert(out, (cond and 'PASS ' or 'FAIL ') .. desc)
+end
+
+-- ----- setup() top-level scalar folds into every wiki; per-wiki wins -----
+config.apply({
+  toc_header_level = 2,
+  html_header_numbering = 2,
+  wikis = {
+    { root = '/tmp/one', name = 'One' },
+    { root = '/tmp/two', name = 'Two', toc_header_level = 3 },
+  },
+})
+local io1 = lsp.init_options()
+check('w0 inherits global level', io1.wikis[1].toc_header_level == 2)
+check('w0 inherits numbering', io1.wikis[1].html_header_numbering == 2)
+check('w1 per-wiki override wins', io1.wikis[2].toc_header_level == 3)
+check('w1 still inherits numbering', io1.wikis[2].html_header_numbering == 2)
+
+-- The user's setup table must not be mutated by the fold.
+check('no mutation of user wikis', config.user.wikis[1].toc_header_level == nil)
+
+-- ----- built-in defaults are NOT folded (only explicit user values) -----
+config.apply({
+  wikis = { { root = '/tmp/x', name = 'X' } },
+})
+local io2 = lsp.init_options()
+check('default toc_header not folded', io2.wikis[1].toc_header == nil)
+check('default level not folded', io2.wikis[1].toc_header_level == nil)
+
+-- ----- g:nuwiki_<key> acts as a global too -----
+config.apply({
+  wikis = { { root = '/tmp/y', name = 'Y' } },
+})
+vim.g.nuwiki_toc_header_level = 5
+local io3 = lsp.init_options()
+check('g:nuwiki_ global folds', io3.wikis[1].toc_header_level == 5)
+vim.g.nuwiki_toc_header_level = nil
+
+local f = assert(io.open(os.getenv('NUWIKI_GS_OUT'), 'w'))
+f:write(table.concat(out, '\n') .. '\n')
+f:close()
+vim.cmd('qa!')