phase 14: list/heading edit commands + CommandOutcome split

Six new executeCommand entries — the cheap surgical edits that account
for the bulk of vimwiki users' daily keymap traffic. Heavier commands
(table.align/moveColumn/insert, list.changeSymbol/changeLevel/renumber/
removeDone, link.normalize, colorize) are deferred for a follow-up
since each needs its own structural rewriter.

CommandOutcome split:
- `Edit(WorkspaceEdit)` — server applies via apply_edit.
- `Value(Value)` — returned verbatim to the client (used by
  read-only queries like nextTask).
- execute_command handler in lib.rs dispatches both cases.

New commands:
- nuwiki.list.toggleCheckbox — [ ] ↔ [X]; mid-states snap to [X];
  rejected reverts to [ ].
- nuwiki.list.cycleCheckbox — [ ] → [.] → [o] → [O] → [X] → [ ];
  rejected resets to [ ].
- nuwiki.list.rejectCheckbox — toggle the [-] rejected marker.
- nuwiki.list.nextTask — returns Location of the first unfinished
  task after the cursor; wraps to the start of the doc when none
  found below. Only items with checkboxes count.
- nuwiki.heading.addLevel — = → ==; clamps at 6.
- nuwiki.heading.removeLevel — == → =; clamps at 1 (no-op on h1).

Pure ops:
- ops::toggle_state / cycle_state / reject_state — checkbox state
  transitions.
- ops::checkbox_edit / next_task / heading_change_level —
  end-to-end edit producers. Tests drive these directly without
  spinning up a Backend.
- ops::rewrite_heading_with_level — text-level rewrite preserving
  the leading whitespace that marks a centered heading (lives
  outside the heading span, so the edit doesn't touch it).
- ops::find_list_item_at / find_checkbox_span / item_in_list —
  AST + source-text helpers, pub for downstream tooling.

Tests (21 new):
- State transitions: round-trip, progression, dash toggle.
- checkbox_edit: empty → done, partial → cycle, reject path,
  no-op on plain item, no-op off list, nested-sublist resolution.
- heading_change_level: h2→h3, level-6 cap, h3→h2, h1 no-op,
  off-heading no-op, span boundary semantics.
- next_task: first-unfinished, wrap-around, all-done returns
  None, plain items ignored.
- COMMANDS list ↔ handler match.

All 211 prior tests still pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/nuwiki-lsp/src/lib.rs

@@ -639,7 +639,7 @@ impl LanguageServer for Backend {
         params: ExecuteCommandParams,
     ) -> LspResult<Option<serde_json::Value>> {
         match commands::execute(self, &params.command, params.arguments).await {
-            Ok(Some(edit)) => {
+            Ok(Some(commands::CommandOutcome::Edit(edit))) => {
                 if let Err(e) = self.client.apply_edit(edit).await {
                     self.client
                         .log_message(
@@ -650,6 +650,7 @@ impl LanguageServer for Backend {
                 }
                 Ok(None)
             }
+            Ok(Some(commands::CommandOutcome::Value(v))) => Ok(Some(v)),
             Ok(None) => Ok(None),
             Err(msg) => {
                 self.client