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>

README.md

@@ -255,6 +255,7 @@ plugin:
 let g:nuwiki_wiki_root            = '~/vimwiki'
 let g:nuwiki_file_extension       = '.wiki'
 let g:nuwiki_log_level            = 'warn'
+let g:nuwiki_link_severity        = 'warn' " 'off'|'hint'|'warn'|'error'
 let g:nuwiki_no_default_mappings  = 0   " set to 1 to skip the keymap layer
 let g:nuwiki_no_folding           = 0   " set to 1 to skip foldexpr setup
 let g:nuwiki_mouse_mappings       = 0   " set to 1 to enable mouse maps
@@ -273,7 +274,7 @@ let g:nuwiki_mouse_mappings       = 0   " set to 1 to enable mouse maps
 | `folding` | string | `'lsp'` | `'lsp'` \| `'expr'` \| `'off'` |
 | `wikis` | list of tables | `nil` | per-wiki tables (see below); wins over the single-wiki shorthand |
 | `mappings` | table | all on | keymap subgroups (see below) |
-| `diagnostic` | table | — | `{ link_severity = … }` (see note below) |
+| `diagnostic` | table | `{ link_severity = 'warn' }` | `link_severity`: `'off'` \| `'hint'` \| `'warn'` \| `'error'` |
 
 #### `mappings` subgroups
 
@@ -340,15 +341,11 @@ For users configuring without Lua.
 | `g:nuwiki_wiki_root` | `'~/vimwiki'` | single-wiki root |
 | `g:nuwiki_file_extension` | `'.wiki'` | wiki file extension |
 | `g:nuwiki_log_level` | `'warn'` | `error` \| `warn` \| `info` \| `debug` |
+| `g:nuwiki_link_severity` | `'warn'` | broken-link severity: `off` \| `hint` \| `warn` \| `error` |
 | `g:nuwiki_no_default_mappings` | `0` | `1` skips the keymap layer |
 | `g:nuwiki_no_folding` | `0` | `1` skips foldexpr setup |
 | `g:nuwiki_mouse_mappings` | `0` | `1` enables mouse maps |
 
-> **Note:** `diagnostic.link_severity` is accepted in the config surface but
-> the server currently always uses its default (`'warn'`); the client value is
-> not yet wired through. Track this if you rely on changing broken-link
-> severity.
-
 ---
 
 ## Usage