Characters: campaigns, "Spielen" main menu, story unlocks

- One world per campaign and difficulty in a character file (sections
  ancaria<d> / underworld<d>; old world<d> renamed when read). Difficulty
  unlocks by the shared hero's story completions (hero header +0xed..),
  shared by both campaigns.
- Main menu: "Kampagne Ancaria", "Kampagne Underworld" and "Spielstaende"
  become "Spielen" (mods/mainmenu.cpp). The load window lists all
  characters, the world dialog offers both campaigns, "Neuer Charakter"
  asks for the campaign and opens character creation.
- Character window: no Underworld import mode (new Underworld heroes, all
  classes, slots light up), no "Importieren"; load window without
  "Speichern".

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

src/mods/charfile.h

@@ -2,6 +2,7 @@
 // Character files (docs/characters.md): one file per character with its shared
 // hero and one world per difficulty, plus reading the game's save files.
 
+#include <array>
 #include <cstdint>
 #include <optional>
 #include <string>
@@ -16,6 +17,11 @@ using Bytes = std::vector<uint8_t>;
 // Bronze, Silber, Gold, Platin, Niob (g_gameLevel 0..4).
 constexpr int kDifficultyCount = 5;
 
+// The campaigns (g_campaign 0x182cdc8): Ancaria 0, Underworld 1.
+constexpr int kCampaignAncaria = 0;
+constexpr int kCampaignUnderworld = 1;
+constexpr int kCampaignCount = 2;
+
 // What the game's save files (GAME.PAK "AMS", hero files .pax "AMH") say about
 // their hero: header 0x100 bytes, then a table of 64 chunks {type, offset,
 // size}; chunk 0xc3 is the hero header.
@@ -23,9 +29,12 @@ struct SaveInfo {
     std::wstring name;                 // hero header +8, UTF-16
     uint32_t heroClass = 0;            // hero header +4: 1 Seraphim .. 9 Daemonin
     uint32_t level = 0;                // hero header +0
+    // Story completions per difficulty (hero header +0xed.., cStatsManager
+    // +0x5704; the hero file's are the hero's, loadHero sets them).
+    std::array<uint8_t, kDifficultyCount> storyCompleted{};
     bool world = false;                // a save game (AMS), not a hero file
     int difficulty = 0;                // AMS header +0x84 (g_gameLevel)
-    uint32_t mode = 0;                 // AMS header +0x24: 2 normal, 3 hardcore
+    int campaign = kCampaignAncaria;   // AMS header +0x24 (CampaignSaveMode)
     bool multiplayer = false;          // AMS header +0x2c
     uint64_t savedAt = 0;              // AMS header +0x5c SYSTEMTIME, as FILETIME
 };
@@ -37,10 +46,15 @@ std::optional<SaveInfo> ReadSaveInfo(const Bytes& file);
 constexpr size_t kHeroNameMaxChars = 63;
 bool SetHeroName(Bytes& file, const std::wstring& name);
 
+// 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);
+
 // A small save game the game's load window can list: `world`'s header with its
 // date, its thumbnail and world time, and `description` (ANSI, up to 63
-// characters) as the save's name.
-Bytes BuildPreview(const Bytes& world, const std::string& description);
+// characters) as the save's name. The load window lists only saves of the
+// running campaign (0x71e2b9): the preview names `campaign`.
+Bytes BuildPreview(const Bytes& world, const std::string& description, int campaign);
 
 // Reading and writing whole files (paths in the game's code page).
 std::optional<Bytes> ReadFile(const std::string& path);
@@ -77,6 +91,8 @@ private:
     std::vector<Section> sections_;
 };
 
-std::string WorldSection(int difficulty);
+// A world's section: "ancaria<d>" / "underworld<d>", d = g_gameLevel. Sections
+// are named, not slotted: more difficulties (or campaigns) are more sections.
+std::string WorldSection(int campaign, int difficulty);
 
 } // namespace mods::characters