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/gfx.rs

@@ -1382,6 +1382,7 @@ impl Gfx {
             Kind::Shard => T_HEROFLOOR + v,
             Kind::Aerie => T_AERIE + v,
             Kind::Steel => T_STEEL + v,
+            Kind::ManaSpring => T_PORTALFLOOR + v,
         }
     }