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/campaign.rs

@@ -9,8 +9,8 @@ use serde::{Deserialize, Serialize};
 
 // From which mission (0-based) on each tool is available (same order as ui::TOOLS):
 // dig, hand, treasury, lair, farm, training, library, sell, flag, lightning, heal, imp,
-// torture, temple, arrow tower, cannon tower, magic tower, bridge, aerie
-pub const TOOL_UNLOCK: [usize; 19] = [0, 0, 0, 0, 0, 2, 4, 0, 0, 3, 4, 0, 6, 9, 3, 5, 9, 1, 6];
+// torture, temple, arrow tower, cannon tower, magic tower, bridge, aerie, steel
+pub const TOOL_UNLOCK: [usize; 20] = [0, 0, 0, 0, 0, 2, 4, 0, 0, 3, 4, 0, 6, 9, 3, 5, 9, 1, 6, 10];
 
 pub struct FortDef {
     pub at: (f32, f32), // centre as a fraction of the map size