feat(13.1-C): link helpers — pasteWikilink, pasteUrl, normalize

Closing out §13.1's smallest cluster. Three commands move from
"not yet implemented" stubs to real behaviour:

- `nuwiki.link.pasteWikilink` (server, executeCommand) — derives
  the current page name from the source URI + wiki root, returns a
  `WorkspaceEdit` inserting `[[<page>]]` at the requested cursor
  position.
- `nuwiki.link.pasteUrl` (server) — same lookup, but the inserted
  text is the page's relative HTML output URL
  (`<page>.html`, including subdir segments) so the snippet survives
  when the export root moves.
- `nuwiki.link.normalize` (client) — wraps the word at cursor as
  `[[word]]` without following. Reuses the `wrap_cword_as_wikilink`
  helper that already powers the `<CR>` two-step. Pure-VimL on the
  Vim path; pure-Lua on the Neovim path. No LSP round-trip.

Keymaps:
- `+` (normal + visual) now actually calls `normalize_link` on both
  editor paths instead of stubbing with a "deferred" notification.

Tests:
- 5 new Rust unit tests in `cluster_c_link_helpers.rs` covering
  command-list presence + the page-name derivation for root and
  subdirectory pages + the URL / wikilink shape strings.
- Neovim keymap harness gains a `links.normalize_via_+` case (20
  passing now, up from 19). Vim harness inherits the same
  command-presence check via its existing smoke tests.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ftplugin/vimwiki.vim

@@ -129,8 +129,8 @@ if !has('nvim')
     nnoremap <silent><buffer> <BS>       <C-o>
     nnoremap <silent><buffer> <Tab>      /\[\[<CR>:nohlsearch<CR>
     nnoremap <silent><buffer> <S-Tab>    ?\[\[<CR>:nohlsearch<CR>
-    nnoremap <silent><buffer> +          :echohl WarningMsg<bar>echom 'nuwiki: :VimwikiNormalizeLink not yet implemented — see SPEC §13.1'<bar>echohl None<CR>
-    xnoremap <silent><buffer> +          :<C-u>echohl WarningMsg<bar>echom 'nuwiki: :VimwikiNormalizeLink not yet implemented — see SPEC §13.1'<bar>echohl None<CR>
+    nnoremap <silent><buffer> +          :call nuwiki#commands#normalize_link()<CR>
+    xnoremap <silent><buffer> +          :<C-u>call nuwiki#commands#normalize_link()<CR>
     nnoremap <silent><buffer> <Leader>wn :call nuwiki#commands#wiki_goto_page('')<CR>
     nnoremap <silent><buffer> <Leader>wd :call nuwiki#commands#delete_file()<CR>
     nnoremap <silent><buffer> <Leader>wr :call nuwiki#commands#rename_file()<CR>