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>

lua/nuwiki/keymaps.lua

@@ -184,8 +184,8 @@ function M.attach(bufnr, mappings)
     map('n', '<BS>',    '<C-o>', { desc = 'nuwiki: go back', remap = true }, bufnr)
     map('n', '<Tab>',   link.next, { desc = 'nuwiki: next wikilink' }, bufnr)
     map('n', '<S-Tab>', link.prev, { desc = 'nuwiki: prev wikilink' }, bufnr)
-    map('n', '+',       deferred(':VimwikiNormalizeLink'), { desc = 'nuwiki: normalize link (deferred)' }, bufnr)
-    map('x', '+',       deferred(':VimwikiNormalizeLink'), { desc = 'nuwiki: normalize link (deferred)' }, bufnr)
+    map('n', '+',       cmd.normalize_link, { desc = 'nuwiki: wrap word as wikilink' }, bufnr)
+    map('x', '+',       cmd.normalize_link, { desc = 'nuwiki: wrap word as wikilink' }, bufnr)
     map('n', '<Leader>wn', cmd.wiki_goto_page, { desc = 'nuwiki: goto page' }, bufnr)
     map('n', '<Leader>wd', cmd.delete_file,    { desc = 'nuwiki: delete page' }, bufnr)
     map('n', '<Leader>wr', cmd.rename_file,    { desc = 'nuwiki: rename page' }, bufnr)