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>

crates/nuwiki-lsp/tests/commands_checkboxes.rs

@@ -47,6 +47,24 @@ fn cycle_state_walks_progression() {
     assert_eq!(ops::cycle_state("[-]"), Some("[ ]"));
 }
 
+#[test]
+fn cycle_state_back_is_exact_inverse() {
+    // `glp` walks the progression in reverse — every step must undo the
+    // matching `cycle_state` step.
+    assert_eq!(ops::cycle_state_back("[ ]"), Some("[X]"));
+    assert_eq!(ops::cycle_state_back("[X]"), Some("[O]"));
+    assert_eq!(ops::cycle_state_back("[O]"), Some("[o]"));
+    assert_eq!(ops::cycle_state_back("[o]"), Some("[.]"));
+    assert_eq!(ops::cycle_state_back("[.]"), Some("[ ]"));
+    assert_eq!(ops::cycle_state_back("[-]"), Some("[ ]"));
+    // Composing forward then backward returns to the start for every
+    // in-cycle marker.
+    for s in ["[ ]", "[.]", "[o]", "[O]", "[X]"] {
+        let fwd = ops::cycle_state(s).unwrap();
+        assert_eq!(ops::cycle_state_back(fwd), Some(s));
+    }
+}
+
 #[test]
 fn reject_state_toggles_dash_marker() {
     assert_eq!(ops::reject_state("[ ]"), Some("[-]"));