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>

autoload/nuwiki/commands.vim

@@ -480,9 +480,25 @@ endfunction
 function! nuwiki#commands#colorize(color) abort
   call s:notify_deferred(':VimwikiColorize')
 endfunction
+" §13.1 Cluster C — link helpers.
+
 function! nuwiki#commands#paste_link() abort
-  call s:notify_deferred(':VimwikiPasteLink')
+  let l:p = s:cursor_position()
+  let l:args = [{ 'uri': l:p['textDocument']['uri'], 'position': l:p['position'] }]
+  call s:exec('nuwiki.link.pasteWikilink', l:args)
 endfunction
+
 function! nuwiki#commands#paste_url() abort
-  call s:notify_deferred(':VimwikiPasteUrl')
+  let l:p = s:cursor_position()
+  let l:args = [{ 'uri': l:p['textDocument']['uri'], 'position': l:p['position'] }]
+  call s:exec('nuwiki.link.pasteUrl', l:args)
+endfunction
+
+" `:VimwikiNormalizeLink` — wrap the word at cursor as `[[word]]`
+" without following. Pure-VimL.
+function! nuwiki#commands#normalize_link() abort
+  if s:cursor_inside_wikilink()
+    return
+  endif
+  call s:wrap_cword_as_wikilink()
 endfunction