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>

src/mods/charfile.h

@@ -46,6 +46,11 @@ std::optional<SaveInfo> ReadSaveInfo(const Bytes& file);
 constexpr size_t kHeroNameMaxChars = 63;
 bool SetHeroName(Bytes& file, const std::wstring& name);
 
+// Sets the hero header's story completions of `difficulty` (+0xed..; in a hero
+// file they are the hero's, loadHero puts them into cStatsManager); false if
+// the file has no hero header.
+bool SetStoryCompleted(Bytes& file, int difficulty, uint8_t count);
+
 // The save header's campaign field (+0x24): 2 Ancaria, 3 Underworld (written
 // at 0x61a5c5, read back by cEngine::Load 0x61bafd).
 uint32_t CampaignSaveMode(int campaign);