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/render3d.rs

@@ -1717,6 +1717,7 @@ pub fn minimap_color(tile: &Tile) -> [u8; 4] {
         Kind::LavaBridge => [130, 120, 115, 255],
         Kind::Shard => [255, 60, 120, 255],
         Kind::Aerie => [170, 140, 90, 255],
+        Kind::Steel => [150, 160, 180, 255],
     }
 }