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.cpp

@@ -123,6 +123,16 @@ bool SetHeroName(Bytes& file, const std::wstring& name) {
     return true;
 }
 
+bool SetStoryCompleted(Bytes& file, int difficulty, uint8_t count) {
+    const std::optional<Chunk> hero = FindChunk(file, kChunkHero);
+    if (!hero || difficulty < 0 || difficulty >= kDifficultyCount ||
+        hero->size < kHeroStoryCompleted + kDifficultyCount) {
+        return false;
+    }
+    file[hero->offset + kHeroStoryCompleted + difficulty] = count;
+    return true;
+}
+
 uint32_t CampaignSaveMode(int campaign) {
     return campaign == kCampaignUnderworld ? kSaveModeUnderworld : kSaveModeAncaria;
 }