test(config): cover link_severity in parity harnesses and docs

Both editor harnesses now emit diagnostic.link_severity and exercise a
non-default override ('error') in the sample scenario, diffed against the
shared golden. Drop the README "not wired" caveat now that the value flows
through, and document the g:nuwiki_link_severity global.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

development/tests/config-expected.txt

@@ -3,11 +3,13 @@ wiki_root=~/vimwiki
 file_extension=.wiki
 syntax=vimwiki
 log_level=warn
+diagnostic.link_severity=warn
 [sample]
 wiki_root=/tmp/base
 file_extension=.md
 syntax=vimwiki
 log_level=debug
+diagnostic.link_severity=error
 wikis[0].diary_rel_path=journal
 wikis[0].file_extension=.wiki
 wikis[0].index=home

development/tests/test-config-vim.vim

@@ -15,6 +15,9 @@ function! s:dump(payload) abort
   call add(s:lines, 'file_extension=' . a:payload.file_extension)
   call add(s:lines, 'syntax=' . a:payload.syntax)
   call add(s:lines, 'log_level=' . a:payload.log_level)
+  if has_key(a:payload, 'diagnostic')
+    call add(s:lines, 'diagnostic.link_severity=' . a:payload.diagnostic.link_severity)
+  endif
   if has_key(a:payload, 'wikis')
     let l:i = 0
     for l:w in a:payload.wikis
@@ -28,7 +31,7 @@ endfunction
 
 " Scenario 1: defaults (no globals set).
 unlet! g:nuwiki_wiki_root g:nuwiki_file_extension g:nuwiki_syntax
-unlet! g:nuwiki_log_level g:nuwiki_wikis
+unlet! g:nuwiki_log_level g:nuwiki_wikis g:nuwiki_link_severity
 call add(s:lines, '[default]')
 call s:dump(nuwiki#lsp#settings())
 
@@ -37,6 +40,7 @@ let g:nuwiki_wiki_root      = '/tmp/base'
 let g:nuwiki_file_extension = '.md'
 let g:nuwiki_syntax         = 'vimwiki'
 let g:nuwiki_log_level      = 'debug'
+let g:nuwiki_link_severity  = 'error'
 let g:nuwiki_wikis = [
       \ { 'name': 'personal', 'root': '/tmp/personal',
       \   'file_extension': '.wiki', 'index': 'home',

development/tests/test-config.lua

@@ -27,6 +27,9 @@ local function dump(payload)
   emit('file_extension=' .. tostring(payload.file_extension))
   emit('syntax=' .. tostring(payload.syntax))
   emit('log_level=' .. tostring(payload.log_level))
+  if payload.diagnostic then
+    emit('diagnostic.link_severity=' .. tostring(payload.diagnostic.link_severity))
+  end
   if payload.wikis then
     for i, w in ipairs(payload.wikis) do
       local keys = {}
@@ -53,6 +56,7 @@ config.apply({
   file_extension = '.md',
   syntax = 'vimwiki',
   log_level = 'debug',
+  diagnostic = { link_severity = 'error' },
   wikis = {
     {
       name = 'personal',