Characters: one file per character with a world per difficulty

- CharacterNN.sacredK holds the shared hero and one world per difficulty,
  plus an autosave file; the game works in save\characters\_work
- Load window: one row per character, a difficulty dialog on Laden
  (worlds, newer autosaves, "(neu)" starts a new game with the shared
  hero), Umbenennen, delete moves the character to characters\deleted
- The world keeps its own portals and hero position when the shared hero
  is put on
- K and the Esc menu's "Speichern" save directly (quicksave path)
- One-time migration of the old save rows and exported heroes, after a
  backup
- Startup no longer hangs in the game's foreground loop (0x810af0)
- Text overrides apply to our own text ids too; shared cUI_Button2
  helpers in game/ui/button.h

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

CMakeLists.txt

@@ -71,6 +71,8 @@ add_library(sacredK SHARED
     src/mods/options.cpp
     src/mods/lootfilter.cpp
     src/mods/userfolders.cpp
+    src/mods/charfile.cpp
+    src/mods/characters.cpp
 )
 target_include_directories(sacredK PRIVATE src)
 target_compile_options(sacredK PRIVATE ${SACREDK_CXX_FLAGS})