Steel walls: plate earth walls next to own ground with steel

- New tool Steel (key O, orders category; campaign from mission 11): 80 gold per earth wall,
  40 for a reinforced wall; riveted iron plate texture
- Heroes dig through steel 20 times slower than through earth and avoid it (path cost 300)
- Selling turns a steel wall back into a reinforced wall and refunds half the plating
- Texts EN/DE, test

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

src/map.rs

@@ -58,6 +58,7 @@ pub enum Kind {
     Ruby,       // ruby vein: rare, valuable
     Diamond,    // diamond vein: very rare, very valuable
     Aerie,      // room for flying creatures (their beds)
+    Steel,      // steel wall: hardly anyone gets through
 }
 
 // options for the map generator (campaign missions)
@@ -86,7 +87,7 @@ pub struct River {
 }
 
 // order matters for the packed savegame codec: only append new kinds
-const KINDS: [Kind; 24] = [
+const KINDS: [Kind; 25] = [
     Kind::Rock,
     Kind::Earth,
     Kind::Gold,
@@ -111,6 +112,7 @@ const KINDS: [Kind; 24] = [
     Kind::Ruby,
     Kind::Diamond,
     Kind::Aerie,
+    Kind::Steel,
 ];
 
 // treasures (veins, treasury content, imp loads): 0 gold, 1 silver, 2 ruby, 3 diamond
@@ -159,10 +161,10 @@ impl Tile {
         Tile { kind, owner: Owner::None, hp: 0.0, gold: 0, marked: false, reserved: 0, food: 0.0, bed: 0, var: 0, reinforced: false, res: 0 }
     }
     pub fn solid(&self) -> bool {
-        matches!(self.kind, Kind::Rock | Kind::Earth | Kind::Gold | Kind::Silver | Kind::Ruby | Kind::Diamond)
+        matches!(self.kind, Kind::Rock | Kind::Earth | Kind::Gold | Kind::Silver | Kind::Ruby | Kind::Diamond | Kind::Steel)
     }
     pub fn diggable(&self) -> bool {
-        matches!(self.kind, Kind::Earth | Kind::Gold | Kind::Silver | Kind::Ruby | Kind::Diamond)
+        matches!(self.kind, Kind::Earth | Kind::Gold | Kind::Silver | Kind::Ruby | Kind::Diamond | Kind::Steel)
     }
     // the treasure of a vein (None for everything else)
     pub fn vein(&self) -> Option<u8> {
@@ -637,6 +639,7 @@ impl Map {
             let tl = &self.t[i];
             match tl.kind {
                 Kind::Earth if tl.reinforced => Some(70),
+                Kind::Steel => Some(300),
                 Kind::Earth => Some(7),
                 Kind::Gold | Kind::Silver | Kind::Ruby | Kind::Diamond => Some(12),
                 Kind::Rock | Kind::Core | Kind::Tower | Kind::Water | Kind::Lava => None,