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>

scripts/test-keymaps.lua

@@ -237,6 +237,17 @@ vim.defer_fn(function()
     wait_ms = 200,
   })
 
+  -- ===== Link helpers (§13.1 Cluster C) =====
+
+  run('links.normalize_via_+', {
+    lines = { 'MyPage rest' },
+    cursor = { 1, 3 },
+    keys = '+',
+    expect_line = '[[MyPage]] rest',
+    expect_at = 1,
+    wait_ms = 200,
+  })
+
   -- ===== Bullet continuation (o/O) =====
 
   run('lists.o_continues_list_marker', {