Campaign of 30 missions: six against the rival masters Mossgrim and Lady Nightwell, islands, mana springs, sealed caves with chained creatures

- missions interleaved into one story (EN/DE), new epilogue, map sizes grow to 176x176
- islands in water or lava with treasure and/or a mana spring; rivers at least four tiles wide
- sealed caves with treasure and a chained creature that joins once the cave is claimed
- new goals: rival cores, mana springs, freed creatures
- rival messages name the masters; new progress key for the new mission order

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/map.rs

@@ -59,6 +59,7 @@ pub enum Kind {
     Diamond,    // diamond vein: very rare, very valuable
     Aerie,      // room for flying creatures (their beds)
     Steel,      // steel wall: hardly anyone gets through
+    ManaSpring, // crystal spring: once claimed, more mana and a higher mana maximum
 }
 
 // options for the map generator (campaign missions)
@@ -87,7 +88,7 @@ pub struct River {
 }
 
 // order matters for the packed savegame codec: only append new kinds
-const KINDS: [Kind; 25] = [
+const KINDS: [Kind; 26] = [
     Kind::Rock,
     Kind::Earth,
     Kind::Gold,
@@ -113,6 +114,7 @@ const KINDS: [Kind; 25] = [
     Kind::Diamond,
     Kind::Aerie,
     Kind::Steel,
+    Kind::ManaSpring,
 ];
 
 // treasures (veins, treasury content, imp loads): 0 gold, 1 silver, 2 ruby, 3 diamond
@@ -207,6 +209,7 @@ impl Tile {
                 | Kind::LavaBridge
                 | Kind::Shard
                 | Kind::Aerie
+                | Kind::ManaSpring
         )
     }
     // river tiles (open, but nobody can walk there)