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>

docs/todo.md

@@ -166,3 +166,33 @@ The hero sometimes plays a walking animation after a save game load while
 standing still. Not reproduced yet: a load log (hero mode, action, animation
 fields, orders) showed nothing unusual; right after a load the game itself
 sends one walk order (0x4) while the hero is idle. Measure again when it shows.
+
+## Multiplayer with characters (2026-10-19)
+
+Characters became one file each with a world per difficulty
+(`docs/characters.md`); vanilla multiplayer picks exported `Heronn.pax`
+heroes, which no longer exist. Later: give multiplayer the shared hero of a
+character file (the session's hero file +0x3a3c, as `SelectHeroSlot` does),
+and decide what a multiplayer game does to the character's progress.
+
+## Characters: open parts of the rework (2026-10-20)
+
+From the characters rework (`docs/characters.md`, request of 2026-10-19):
+
+- **Unlock difficulties by story completion.** Bronze and Silber stay open;
+  difficulty d >= 2 opens when the story on d - 1 is completed. The counters
+  are one byte per difficulty in cStatsManager +0x5704 (read 0x424be0) and
+  travel in the save header (+0x88..+0x98) and the hero file. Today every
+  difficulty up to the highest played one is offered (and Silber).
+- **Reset a difficulty.** Remove one world from a character (the shared hero
+  stays); a button / dialog entry in the load window.
+- **Remove the export / import UI.** The Esc menu's "Exportieren" and the
+  character screen's import (cUI_Character mode 5, `docs/char-export.md`):
+  characters replace exported heroes.
+- **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.
+- **Quickload (row 0) does not know characters.** Ctrl+Z saves into the
+  current character, but the quickload event (0x116, engine +0x68 = row,
+  flag 0x200) loads `GAME00.PAK`, which is not listed; make it reload the
+  current character's world (or its newer autosave).