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>

lang/en.lang

@@ -146,7 +146,7 @@ help.k_slap = Shift+Left      Hand: backhand own units, punch heroes on your gro
 help.k_pan = Right drag      Move the map (also WASD/arrows, screen edge)
 help.k_rot = Middle drag     Rotate and tilt the camera (also Q/E, R/F)
 help.k_zoom = Mouse wheel     Zoom
-help.k_tools = 1-9,0,X,V,K,J,B,C,T,U,Y  Select tool
+help.k_tools = 1-9,0,X,V,K,J,B,C,T,U,Y,O  Select tool
 help.k_misc = P / Space       Pause      M  Sound on/off   +/-  Volume
 help.k_misc2 = G               Tile outlines on/off     H  This help
 help.s1 = 1. Dig to the portal and let your Pit Imps claim it.
@@ -530,3 +530,12 @@ unit.wyrm = Ember Wyrm
 job.rest = Recovering in the aerie
 job.torment = Working on a prisoner
 menu.license = DungeonCraft - MIT License - (c) 2026 Richard Zink
+
+# steel walls
+tool.steel.label = Steel
+tool.steel.name = Steel Walls
+tool.steel.desc = Plates earth walls next to your ground with steel. Heroes hardly get through and rather take long detours. Selling turns them back into reinforced walls.
+cost.steel = {0} gold/wall, {1} for reinforced walls
+hint.steel_where = Steel can only be put on earth walls next to your own ground.
+tile.steel = Steel wall
+tile.steel_info = Heroes need ages to get through.