Fix glp backward cycle and gl<Space> current-list scope

Two documented checkbox-list mappings did not match their spec:

- `glp` dispatched the same forward-only cycleCheckbox as `gln`, so it
  could never "cycle the checkbox state backward". Add `ops::cycle_state_back`
  and thread a `reverse` flag through the dispatcher and both clients.

- `gl<Space>` and `gL<Space>` both swept the whole buffer, making them
  identical. `gl<Space>` now passes the cursor position and the server
  scopes deletion to the contiguous list block under the cursor (current
  list, cascading into sublists); `gL<Space>` stays whole-buffer.

Adds Rust unit tests, plus behavioral and full-registration coverage for
the documented mapping surface in the Neovim and Vim keymap harnesses, and
a command-coverage suite mirroring the doc's command groups.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

scripts/test-keymaps-vim.sh

@@ -33,6 +33,9 @@ VIMRC="$TMP/vimrc"
 cat > "$VIMRC" <<EOF
 set nocompatible
 let &runtimepath = '$REPO_ROOT' . ',' . &runtimepath
+" Enable the opt-in mouse mappings before the ftplugin loads so the
+" harness can assert the full documented mapping surface (doc §6 mouse).
+let g:nuwiki_mouse_mappings = 1
 filetype plugin indent on
 syntax enable
 " ftdetect/nuwiki.vim already maps *.wiki → vimwiki via \`set filetype\`.