feat: time-aware due dates, persistent undo, @return audit

Problem: Due dates had no time component, the undo stack was lost on
restart and stored in a separate file, and many public functions lacked
required @return annotations.

Solution: Add YYYY-MM-DDThh:mm support across parse, views, recur,
complete, and init with time-aware overdue checks. Merge the undo stack
into the task store JSON so a single file holds all state. Add @return
nil annotations to all 27 void public functions across every module.

doc/pending.txt

@@ -35,7 +35,7 @@ Features: ~
   names, month names, ordinals, and more
 - Recurring tasks with automatic next-date spawning on completion
 - Two views: category (default) and priority flat list
-- Multi-level undo (up to 20 `:w` saves, session-only)
+- Multi-level undo (up to 20 `:w` saves, persisted across sessions)
 - Quick-add from the command line with `:Pending add`
 - Quickfix list of overdue/due-today tasks via `:Pending due`
 - Foldable category sections (`zc`/`zo`) in category view
@@ -242,7 +242,7 @@ COMMANDS                                                    *pending-commands*
 :Pending undo
     Undo the last `:w` save, restoring the task store to its previous state.
     Equivalent to the `U` buffer-local key (see |pending-mappings|). Up to 20
-    levels of undo are retained per session.
+    levels of undo are persisted across sessions.
 
 ==============================================================================
 MAPPINGS                                                    *pending-mappings*