feat: real range/bang/visual handling for the deferred command gaps

Implements the four items previously left as "needs handler work" — all
fully wired, not cosmetic.

- ToggleListItem / IncrementListItem / DecrementListItem are now -range
  (both clients). The client loops the per-line op over [<line1>,<line2>]
  (toggle_list_item_range / list_cycle_symbol_range). The server's edits are
  version-less single-line WorkspaceEdits, so each line applies independently
  — no server protocol change. `:'<,'>VimwikiToggleListItem` toggles the whole
  selection.
- VimwikiRebuildTags gains -bang: `:…RebuildTags!` passes { all: true } and
  the server (tags_rebuild) re-indexes every configured wiki via
  wikis_snapshot(), not just the current one.
- VimwikiNormalizeLink is -nargs=?: with `1` (upstream's visual flag; the
  x-mode `+` mapping now passes it) it wraps the '< / '> selection as
  [[selection]] (new wrap_visual_as_wikilink in both clients).
- VimwikiCheckLinks is -range: a ranged invocation filters the broken-link
  report to the current buffer's selected lines (client-side filter in
  results_to_qf / check_links).

Tests: cmd.toggle_list_item_range + cmd.normalize_link_visual_wraps_selection
(test-keymaps.lua), normalize.visual_wraps_selection (test-keymaps-vim.vim).
README + doc/nuwiki.txt + gap doc updated. fmt/clippy clean; nvim 274,
vim 266+18, all Rust tests + config-parity green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lua/nuwiki/keymaps.lua

@@ -174,7 +174,8 @@ function M.attach(bufnr, mappings)
     map('n', '<Tab>',   link.next, { desc = 'nuwiki: next wikilink' }, bufnr)
     map('n', '<S-Tab>', link.prev, { desc = 'nuwiki: prev wikilink' }, 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('x', '+',       function() cmd.normalize_link(true) end,
+        { desc = 'nuwiki: wrap selection 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)