Characters: reset a difficulty

"Zuruecksetzen" in the load window lists the selected character's worlds
and asks for confirmation. The world goes (a copy in characters\deleted),
with an autosave of it, and the shared hero's story completion of that
difficulty is cleared. The dialog is one generic list (title + entries).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/todo.md

@@ -184,8 +184,6 @@ From the characters rework (`docs/characters.md`, request of 2026-10-19):
   are one byte per difficulty in cStatsManager +0x5704 (read 0x424be0) and
   travel in the save header (+0x88..+0x98) and the hero file. Done
   2026-10-20 (docs/characters.md).
-- **Reset a difficulty.** Remove one world from a character (the shared hero
-  stays); a button / dialog entry in the load window.
 - **Silent autosave every 10 minutes** (a setting later) into the
   autosave file, without the busy dialog, like the direct save
   (`SaveOnTick`) but marked as an autosave.