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-vimwiki-compat-vim.vim

@@ -0,0 +1,78 @@
+" development/tests/test-vimwiki-compat-vim.vim — upstream-vimwiki config
+" drop-in compatibility for the Vim client.
+"
+" Sourced by test-vimwiki-compat-vim.sh under headless Vim. Sets an upstream
+" `g:vimwiki_list` + `g:vimwiki_*` config and asserts nuwiki#lsp#settings()
+" translates it into the server's schema. Writes PASS/FAIL to
+" $NUWIKI_VWC_OUT; the wrapper fails on any FAIL.
+
+let s:out = []
+function! s:check(desc, cond) abort
+  call add(s:out, (a:cond ? 'PASS ' : 'FAIL ') . a:desc)
+endfunction
+
+" ----- upstream vimwiki config (no nuwiki-native vars set) -----
+let g:vimwiki_toc_header_level = 2
+let g:vimwiki_html_header_numbering = 2
+let g:vimwiki_html_header_numbering_sym = ' -'
+let s:personal = {}
+let s:personal.path = '~/.vimwiki/personal_wiki'
+let s:personal.path_html = '~/public_html/personal_wiki'
+let s:personal.template_default = 'default'
+let s:personal.template_ext = '.tpl'
+let s:personal.auto_export = 1
+let s:ifood = {}
+let s:ifood.path = '~/.vimwiki/ifood_wiki'
+let g:vimwiki_list = [s:personal, s:ifood]
+
+let s:cfg = nuwiki#lsp#settings()
+
+call s:check('has wikis list',        has_key(s:cfg, 'wikis'))
+call s:check('two wikis',             has_key(s:cfg, 'wikis') && len(s:cfg.wikis) == 2)
+let s:w0 = get(s:cfg, 'wikis', [{}])[0]
+let s:w1 = get(s:cfg, 'wikis', [{},{}])[1]
+call s:check('w0 path->root',         get(s:w0, 'root', '') ==# '~/.vimwiki/personal_wiki')
+call s:check('w0 path_html->html_path', get(s:w0, 'html_path', '') ==# '~/public_html/personal_wiki')
+call s:check('w0 template_default',   get(s:w0, 'template_default', '') ==# 'default')
+call s:check('w0 auto_export',        get(s:w0, 'auto_export', 0) == 1)
+call s:check('w0 toc_header_level=2', get(s:w0, 'toc_header_level', 0) == 2)
+call s:check('w0 html_numbering=2',   get(s:w0, 'html_header_numbering', 0) == 2)
+call s:check('w0 numbering_sym',      get(s:w0, 'html_header_numbering_sym', '') ==# ' -')
+call s:check('w1 root',               get(s:w1, 'root', '') ==# '~/.vimwiki/ifood_wiki')
+call s:check('w1 globals applied',    get(s:w1, 'toc_header_level', 0) == 2)
+
+" ----- nuwiki-native config wins over vimwiki -----
+let g:nuwiki_wikis = [{'root': '~/native', 'name': 'native'}]
+let s:cfg2 = nuwiki#lsp#settings()
+call s:check('native g:nuwiki_wikis wins',
+      \ len(get(s:cfg2, 'wikis', [])) == 1
+      \ && get(s:cfg2.wikis[0], 'root', '') ==# '~/native')
+unlet g:nuwiki_wikis
+
+" ----- nuwiki-native scalar overrides the vimwiki global -----
+let g:vimwiki_list = []
+let g:nuwiki_toc_header_level = 4
+let s:cfg3 = nuwiki#lsp#settings()
+" No wikis list now (vimwiki_list empty) → scalar folded into top-level.
+call s:check('native scalar override', get(s:cfg3, 'toc_header_level', 0) == 4)
+unlet g:nuwiki_toc_header_level
+
+" ----- global shorthand on native g:nuwiki_wikis (vimwiki-style) -----
+" A global default applies to every wiki; a per-wiki value overrides it.
+let g:nuwiki_toc_header_level = 2
+let g:nuwiki_html_header_numbering = 2
+let g:nuwiki_wikis = [
+      \ {'root': '~/w1', 'name': 'One'},
+      \ {'root': '~/w2', 'name': 'Two', 'toc_header_level': 3},
+      \ ]
+let s:cfg4 = nuwiki#lsp#settings()
+let s:g0 = s:cfg4.wikis[0]
+let s:g1 = s:cfg4.wikis[1]
+call s:check('global folds into wiki 0',  get(s:g0, 'toc_header_level', 0) == 2)
+call s:check('global numbering folds',    get(s:g0, 'html_header_numbering', 0) == 2)
+call s:check('per-wiki override wins',     get(s:g1, 'toc_header_level', 0) == 3)
+call s:check('global still folds elsewhere', get(s:g1, 'html_header_numbering', 0) == 2)
+" The user's original dict must not be mutated by the fold.
+call s:check('no mutation of g:nuwiki_wikis', !has_key(g:nuwiki_wikis[0], 'toc_header_level'))
+
+call writefile(s:out, $NUWIKI_VWC_OUT)